PIN messages are sent from one Blackberry to another, the same way Blackberry Messenger messages are sent (using the PIN), but more like an email. When you select "Compose PIN", you then would select a contact that has their PIN in their entry.
Texts are actually SMS messages that can sent amongst any device on any carrier and are charged based on the sender's and receiver's own individual phone plans in regards to texting.
PIN messages are strictly amongst BlackBerry uses. Both sender and receiver must have data plans for their devices. PINs are generally much faster at sending and receiving than SMSs.
what is the purpose of compose pin if u can just use bbm?
It is useful for people who elect not to install BBM, but need to send a message
to another BB user. It is quite a tedious way to converse but for some it works.