Are there any rules or regulations as to when the term "based on a true story" or "real events" and so on can be used?
Does anything in the movie has to be somehow based on the real world, or is this phrase just a marketing tool and can be placed before any movie?
There are no rules as to when or how the term "Based on a true story/real events" can be used.
Sometimes it is a legit claim, but sometimes it is a pure marketing decision to fool the audience.
Basically you have 4 types of movies that make this claim:
The fake true story
The fictionalized true story
The altered true story
The real true story
An additional case was mentioned by Steve-O in the comments (slightly paraphrased):
Movies that were "based on a true story" but where certain details were fictionalized because producers thought the actual facts were deemed too hard to believe by the audience.
This is actually a variation of the 3rd case: The altered true story.
The normal case is that, to make the movie more appealing, the true event is exaggerated.
But there exists a reversed version, where the true event is made less awesome.
An example of this case can be found in Public Enemies
The scene where John Dillinger escapes jail and takes 3 people hostage with a wooden gun is factually incorrect.
It was actually 17 to 33 people (depending the sources asked. Either the jail administrator or Dillinger himself).
However the director, Michael Mann, believed that this would be too unrealistic.
As mentioned in the comments Fargo is another prime example of the 1st case.
A movie that is advertised as "a true story", but was completely made up.
However this is a nice example for another reason.
The "true story" claim was so successfull that many people believed that the money from the movie was really buried somewhere.
Which lead to the urban legend around Takako Konishi, a Japanese woman who supposedly travelled to the USA and died of exposure to the extreme cold while looking for the buried money.
Because this in turn created a movie of case 3, the altered true story, called Kumiko the Treasure Hunter. Which tells her story, but is largely fictionalized.
Westerns are set in the Wild West, which is more or less an exaggerated version of a real region, the western two thirds of the USA, during a real time, usually the 19th century, and mostly in a few decades between about 1860 and 1900.
And because westerns are more or less historical fiction, historical people, places, things, and events are often mentioned in westerns and sometimes depicted.
And how accurately are historical persons, places, things, and events depicted in western movies and television episodes?
There is a blog called Jeff Arnold's West, which has reviews of hundreds of westerns. Arnold often comments that westerns aren't intended to be educational or true. And when he reviews a movie that claims to be a true story he usually comments that the more a movie claims to be true the less true it is likely to be.
For example Walk the Proud Land (1954) opens with this narration:
“The story you’re about to see is true. It happened the way my father told it to me. It started long before I was born, on a hot dusty afternoon in 1874 when he rolled into Tucson on top of a stagecoach.”
The movie is based on the 1936 book Apache Agent by Woodworth Clum, a biography of his father, John Clum (1851-1932), who was the Indian agent at the San Carlos Reservation in Arizona from August 4, 1874 to July 1, 1877.
The events in the movie seem to happen over only a few weeks of fictional time, even though the historical events it is based on happened over 2 years, 7 months, and 28 days from August 4, 1877 to April 1, 1877.
That might be considered a dramatically necessary time compression, but the movie makes big historical changes like making Eskiminzin (c. 1828-1894) the head Apache chief instead of merely the chief of one Apache group, and having a fictional General Wade in charge of the army in Arizona, making Tucson and San Carlos much closer than in real life, etc., etc.
One might expect that John Clum could have told a somewhat garbled account of his experiences with the Apaches to his son Woodworth many years later, but on the other hand Woodworth could have checked the stories he was told with various books about the Indian Wars published before 1932. I suspect that most of the historical inaccuracies in Walk the Proud Land (1954) are the changes that scriptwriters Gil Doud and Jack Sher made to the story.
As Jeff Arnold says:
It was also historically very dubious, normally OK for a Western but if it’s plugged as a biopic and opens with the pronouncement “The story you are about to see is true”, well, it should be better in that regard.
http://jeffarnoldblog.blogspot.com/search/label/Audie%20Murphy?updated-max=2019-07-25T08:41:00%2B02:00&max-results=20&start=1&by-date=false1
Similarly, White Feather (1955) opens with the narration:
“This is the northern ranges of Wyoming. The year is 1877. What you are about to see really happened. The only difference will be when Indians speak they will speak in our language so you can understand them.”
But every single character in the movie is fictional. Part of the plot involves the Northern Cheyenne moving south to the the Indian Territory in 1877, which is loosely based on historical events, and another part of the plot is based on another historical event that happened 13 years later in 1890, when two young Cheyenne made a date to fight the soldiers and be killed. Naturally the film doesn't give the true names of those 2 young Cheyenne who made an appointment to die, or show one of them as actually being only thirteen years old.
As Jeff Arnold says:
The opening words, spoken by Tanner/Wagner in voiceover, are “What you are about to see actually happened”, a sure sign that we are in for unhistorical melodrama, which is what we get.
http://jeffarnoldblog.blogspot.com/search/label/Robert%20Wagner2
As these examples show, when a western claims to be a true story, It is probably at least ninety percent fictional and no more than ten percent historical, and often much, much less than ten percent true.
I think that films set in recent decades that are allegedly based on true stories would probably have a higher ratio of fact to fiction than westerns that claim to be true stories. But a higher ratio of fact to fiction should not be interpreted as being mostly fact with a tiny little bit of fiction thrown in. I suspect that in most cases the ratio might be 20 percent fact to 80 percent fiction, 40 percent fact to 60 percent fiction, 55 percent fact to 45 percent fiction, etc., etc., etc.
So in my opinion if someone watches a movie that claims to be true or based on a true story and is interested the subject of the movie, it would be a good idea to read about that subject to find out how much truth and how much fiction is in the film.
Or go to a website like History vs Hollywood: http://www.historyvshollywood.com/3
Or maybe check if the movie is discussed in "Based on a True Story" trope in TV Tropes:
https://tvtropes.org/pmwiki/pmwiki.php/Main/BasedOnATrueStory4
There are no real official rules. You don't have a Hollywood Genre Classification Board that has clearly defined rules for each and every genre a movie may use based on the contents and punishes movies if they end up using different genres. Movies being an art form, the decision for what labels to stick on a movie are left to the production team.
However, there can be negative consequences to labeling your movie as something it's not. The biggest risk is that the audience is unhappy that you labeled your movie as a true story while it isn't or mostly isn't and boycotts your movie (boycott here being used as a generic term that also includes encouraging others not to watch it through reviews or word-of-mouth). There is also a smaller risk that, if you take it too far in your advertisement campaign, one or more media watch dog organization declares your marketing campaign to be misleading and punishes you, usually with a fine, an order to stop the campaign and potentially an order to start an additional campaign where you issue a public apology for misleading the consumer.
Think of it as a disclaimer. Its saying, "This work isn't non-fiction, but it's not fiction either." Its trying to absolve itself from all responsibility that writers of either type of work might have.
A non-fictional work has the issue that there are loads of people around who fancy themselves experts on history and/or recent events, and if you get some detail wrong in their eyes, they'd just love to publicly rake you over the coals about it.
Also, if you are attempting to depict an actual person, and that actual person is still living (or their estate is still making money off their name), and they don't like your depiction of them for some reason, they can and will sue you. Even if you'd be likely to win the suit, writers and producers want to write and produce their art, they don't want to spend their days hanging out in courtrooms. They'd like to spend their money on producing their stories, not on lawyers.
A fictional work is expected to be entirely made up. If it in fact contains elements of something that really happened, the producers are likely to get sued. If the writers are forced to admit that in court, they might even lose the case.
So basically, to you it means nothing. Purposely nothing. You are not supposed to have any expectations of the truthfulness or non truthfulness of the story.
