The mark is personal, but doesn't have to always be the same
Arcane Mark has been used countless times on adventure paths as a means to write invisible arcane text and, as finely pointed out by @Ifusaso on his answer, even on recent books. But that doesn't change the fact that it's the personal signature of a spellcaster, and changing that signature makes each signature even less important as they no longer identify the caster.
Being personal means that it's not easy to fake the mark as being another caster's using the Arcane Mark spell, even if you two write exactly the same mark, you would have to make a forgery (more on this in a bit).
This has been pointed out on two occasions by James Jacobs (Paizo's Creative Director), on his own personal "Ask Anything" thread, first here:
1) Can the "personal rune or mark" be changed by the caster at any point in their career, or are they stuck with whatever was picked when the spell was learned?
You can change your rune/mark whenever you want, but the more you do so, the less valuable it becomes as your personal rune and the more it becomes "the scribble of the month."
2)What constitutes a "mark"? Could it be a picture, and if so: how complex?
A mark is basically the fantasy version of a signature. It's a rune, essentially. It could easilly be as complex an ancient Egyptian hieroglyphic, for example.
3) Is an Arcane Mark monochrome?
Only if you want it to be.
4) Is its size fixed, other than by the "one square foot" limitation of the spell?
The spell limitation is all that limits its size. Just as you can sign your name huge or tiny, there is no "official" size for your arcane mark.
5) Are they unique and if so: is there a way to connect a specific mark to a specific caster?
They are not unique, although it's not very classy to knowingly copy someone else's mark, and in some cultures doing so would be considered akin to forgery of a signature.
Then here, clarifying this exact same topic:
Seeing the popular idea of using "Arcane Mark" to writhe "Thief" or "Assassin" and similar words on a target, it is possible to change the text on the fly or you can change it only when you prepare your spells?
I wouldn't necessarily call it a "popular idea," since this is actually the first I've heard anyone doing this. Perhaps popular in a real-world region or a campaign—but remember, the game and its gamers are pretty widespread, and folks generally don't play the game the same. Is this something folks are doing a lot over in the Pathfinder Society? Anyway, the spell arcane mark is not called arcane word—it only allows you to inscribe your personal rune or mark, not any word you want. You can certainly change your personal rune or mark, just as easily as you can change your name, but whether or not the rest of the world acknowledges your name change depends on the world, not you. Frankly, if a spellcaster in my game tried to use this spell to write "thief" on a target (he couldn't write "assassin" since the spell limits your mark to 6 characters), I would start calling him "Thief" instead of whatever the character's real name was, just to drive home the in-game abuse of the spell.
Keep in mind that, by no means, James Jacobs is a rules authority on Pathfinder, but his opinions are considered rules as intended by the majority of the paizo's community.