PIN number is a blackberry's unique indentifier.
It will be used for BB messenger (pin to pin = you to another BB user), and to identify you on the blackberry network.
Signing up on BIS (blackberry internet service, for email etc) will connect your PIN with that account.
If the PIN on a used blackberry is still connected to the previous owner's BIS account, you cant use that for blackberry services, until its released.
Some say they are released, when they aren't.
And some don't say at all.
Others don't know what it is either, and so won't be able to tell you.
Look out for, and ask specifically about whether the PIN is associated with an account already.
"That's it then. Cancel the kitchen scraps for lepers and orphans, no more merciful beheadings... and call off Christmas."