And he would like to know what does a detainer number meant?

Hello l:

I hate to be the bearer of bad news, but a detainer is when some police agency wants to charge him with a crime, and they're going to pick him up when his present sentence is complete.

excon

Detainer means - please detain thie prisoner until we can come and get him, take him into custody and take him back to where he is court ordered to be.

Yes, it means there is a warrant for his arrest somewhere else, and on the date he is to be discharged, he will be detained and picked up by the other agency.

He needs an attorney now to find out what these charges are and to try and perhaps even plea with them out where his current time may count.