Could someone please explain what the last part of this record means?

http://www.couchgenweb.com/civilwar/10thinfa.html

EDWARDS, WILLIAM Pvt - Enl 10 Jul 1861 at Springfield, AR. Disch 15 Jul 1862 at Camp Price, MS. POW sent to MP at St Louis, MO Nov 1864.

As a prisoner, was William "military police" at the prison in St Louis? If so, what would his duties have been? And did this happen to POWs very often?