Shadowrun - The Titans of Old
System Identification Number or SIN refers to the method in which the authorities tracks the activities of its registered citizens. These activities include criminal violations, licenses, legal business transactions, purchasing property, and similar activities. Because it ties together all of the information associated with a particular identity, a SIN is one part birth certificate, one part passport, one part credit history, and one part legal record.
There exist individuals who can forge fakes, but these are often of varying quality and only the most exceptional of forgeries can hold up to determined testing.