A detainer is just a "hold" placed on someone that is in jail. If a subject is incarcerated and has a warrant out of another jurisdiction, then a detainer is placed on that subject so he won't be released before there's a chance for extradition.A detainer is a warrant placed against an inmate for pending charges from another jurisdiction.
These pending charge(s) are usually from some county, federal and/or out-of-state law enforcement agencies.
A detainer warrant may also be placed against an inmate who has had concurrent or consecutive sentences imposed in other jurisdictions.
A third type of detainer one in which a jurisdiction wants to be notified of an inmate's pending release.
A detainer may stem from a state charge or even a state investigation in which no charges have even been filed. A state agency normally notifies the Bureau of Prisons or a national computer database (NCIC) that a matter is pending, regardless of the progress of the matter to that point. Thus that agency is notifying the world that as soon as the federal system is done with that person, this agency wants him or her. The Bureau of Prisons then treats the detainer the same way that they treat a pending charge.
Defendants serving federal sentences should not trust their own recollection of whether there is or might be a pending charge or detainer. There are two reasons for this: Sometimes, even if the inmate knows that the charge was dropped, it could still remain in the system as if it had not been. Additionally, in some circumstances, there are charges pending that a defendant may not even have been aware of. Finally, the BOP’s computer system could just show a pending charge or detainer by mistake. It does happens, and when it does, the inmate does not get his or her sentence credits.
https://www.inmateaid.com/ask-the-inmate/today-i-looked-up-my-husbands-case-online-to-see-if-there-was-any-additional-information-or-if-anything-had-changed-well-when-i-scrolled-down-where-it-said-detainerwarrant-it-said-he-had-one-and-it-was-dated-on-12-29-2014-i-was-wondering-what-does-it-mean#answer
Accepted Answer Date Created: January 02,2015