People often think that a SIN is like a passport, drivers license and generic ID all rolled up into one. That's not quite right. SIN numbers do the same job as all those things, but they do them in a very different way.
For starters, a SIN number is just that: a number. Its not a physical thing, its not even a file on your comlink as such. Its just a reference number. You could fake a SIN number good enough to fool the human eye by simply writing down a long number with a pen. There is no actual information directly stored in the SIN number itself, as such.
What happens when someone checks your ID is this:
1) You give them your SIN number
2) They use that number to check the records, either their own (in the case of big corporations) or the central SIN database.
3) The records will contain information that the person can then use to confirm your identity, from basic stuff like gender and race, to DNA records and other biometric data.
There is a couple of caveats to this. One, the SIN number is stored on your comlink using some sort of magical and utterly unexplained technology, and the specific mechanics of how the above process happens are never really detailed. It just works. Two, as far as I am aware its never really explained where the main SIN number register is, or who administers it. I think its a government thing, but it might be someone bigger like Zurich Orbital, or even the Corporate Court. Either way, its universal: every country in the world uses it, and it works pretty much the same wherever you are.
What is probably of more interest to you guys as Shadowrunners is how a fake SIN works. Like I said before, its a lot more complicated than real world ID's. Its not just a case of getting a photo, and faking up some realistic documentation, for example. To make a fake SIN, you need to make a fake entry into the SIN registry, and then couple it up with some fake but convincing data. This is why fake SINs have ratings; a rating 1 fake SIN is just an entry in the SIN registry with some basic data entered in there, whereas a rating 6 fake SIN would be a full registry entry, including biometric data that matches up with the person the SIN is made for, as well as a whole faked data trail and history, with things like faked shopping habits, fake addresses and fake job history. A basic check, like you might get if you were trying to buy beer and they wanted to check your age for example, would just look you up in the registry and check your age, so a rating 1 SIN would be fine. If, on the other hand, you had been arrested by the cops and were applying for bail, they would do a more in depth check, comparing DNA and looking into your history, so you would need a much better SIN to convince them you were someone else.
One of the other things this means to you guys is that you cant really just swap SINs around with each other. When a SIN is made, it is made for a specific person. You could *maybe* get away with it for a rating 1 SIN, but beyond that any check would instantly show that you were not the person that the SIN number belongs to.
I've been fairly loose with the way SINs work so far, as I didn't want to hit you with too many complications in one go, but I think you understand the world enough now to wrap your head around this

Any questions?