The Student ID number (also referred to as the Scholar ID in some places) is ScholarPack-only unique reference number that is automatically assigned to every student added to the ScholarPack service.
Having a unique number assigned when the student profile is created means that even if a student does not yet have an official UPN, they will still be individually coded on ScholarPack, and so you'll never have to rely on just checking a name when confirming which student profile you're dealing with.
It can be seen on the front page of the student profile or on the extended tab:
