Industry Standard (?)
Okay, brief reality check. While the point of this book (and the object of a lot of worry) is creating scripts in the "right" format, there's no such thing. That's right, there is no such thing as THE correct format. That's why every book on formatting has slightly different formatting rules.
BUT (a big but), there is proper format.
What does this mean? Simply, that there's not just one way to format scripts, but a range of ways that are acceptable. Some books say that a film script's left margin should be 1.5". Others say that it should be 1.7" Both of those are right. Nobody's going to look at your script, pull out a ruler and scream, "One point six-four inches! You'll never work in this town again!" But a 1" left margin... that's not right. That's out of the acceptable range and could piss off a producer who thinks you're trying to trick him into thinking your script is shorter than it really is.
Why is there a range and not a single way? Who knows. Individual preference, often. Changing tastes over time, usually. But sometimes it's a more obnoxious reason. Some TV shows have a unique format just so they can tell if a writer submitting to that show has done her homework. If the writer's script is not in that unique format, the producers can say to themselves, "Well, we're obviously not important enough for the writer to have checked to see how WE do it! No thank you." It's petty, but it's true. Don't worry, though, we'll tell you how to avoid that situation.
Like most rules, the ones in here are meant to be used whenever possible, but broken when necessary. None of this is etched in stone, but you can't go wrong by following it religiously. If you have a really good reason for doing it differently; if that rule-breaking formatting choice tells your story better than anything we describe... go for it. Just don't be cavalier and think that you're story is unique and, therefore NEEDS unique formatting. Remember, the odds are that the greatest scripts you can think of were written using these rules, not some weird, random ones.
Good places to go next: Scripts are elements or Show it don't tell it.