Probably what you call a 'badge' number.  You have a store employee number.  They'll show you how to clock in because they'll have to make sure your finger print is on record.

You also have your EUID, which is your Enterprise ID # of your initials and a number. That's used online to check your schedule, access your payroll stubs and do computerized training. 

You should have a chance to sign up for the union when you do your hire paperwork.  Otherwise you can talk to a shop steward.  I didn't join right away.  It's perhaps best not to.

I already know it's not. I'm leaving to go back to school at the end of the summer