BlackBerry devices all have a unique ID, otherwise known as a PIN (Personal Identification Number). A PIN can be used to send secure messages to other BlackBerry users.
The PIN is a string of 8 alphanumeric characters that is hard coded into your BlackBerry, and cannot be changed. The BlackBerry Internet Service (BIS) recognizes your BlackBerry by its PIN, so it knows where to deliver your email messages. BlackBerry Messenger (BBM) uses the PIN protocol to deliver messages to other BlackBerry users.
PIN Messaging is simply sending a message using the BlackBerry PIN protocol from one BlackBerry directly to another BlackBerry. PIN messages are more secure than email messages, because they are encrypted and only travel from one BlackBerry to another via the cellular networks. They do not traverse the internet. PIN messages appear in the BlackBerry Messaging application along with email messages.
If you have friends on BBM that you would like to send direct PIN messages to, you can retrieve their PIN from their BBM contact. If you have your BBM contact in your BlackBerry's Contacts, you can link it to their BBM contact so that you can send them PIN messages straight from the BlackBerry Contact list.
If you choose to give out your BlackBerry’s PIN, remember that it cannot be changed, so keep your BlackBerry's security in mind and only give your PIN to people you trust.