A SIN is, technically spoken, a number (or more accuratly, probably an alphanumeric character string). I like to think about it as an "identification number", like the number on your passport today. It is, of course, used as an id to store data about you into a database. For example, if a shop sells you something, they will insert an entry into the shop's database like "Sold: X and Y to SIN Z.". Or if you get jailed, the government will insert an entry into your criminal record (which itself consists only of entries in a government database) like "SIN X convicted for Y and Z." (simplified, of course).
The big question is: How much information does this number include in itself?
For example, an identification number on a german identity card (21 digits) contains the birth date, the expiry date, nationality, authority code and some error checking numbers.
So in Shadowrun one could argue, that an ID that is primarily transmitted electronically could be much longer and thus contain much more data in itself. For example, the SIN could contain a hash of the retina (a, let's say, 32 digit number that can be computed from the data gained by scanning your retina - and stays the same every time you scan - and theoretically, it should be impossible to calculate what the retina looks like when only having the hash).
Is there information about what IS stored in a SIN itself? What can you check with a given ID if you don't have access to any government, etc. database?