A Prisoner is a unit that has been captured by another side. The side that has captured them can manifest or remove shackles from them at will. It is even possible for the capturing side to give a prisoner orders, though the lack of Loyalty presumably makes it easy for the captive to disobey. If a prisoner happens to be a commander and is left unguarded and unconfined, they are free to become a fugitive.
Prisoners cannot normally receive Thinkagrams.