It worked like this -- you were captured, taken to a camp, and then paroled in pretty short order (usually within 2 weeks). To get paroled you had to promise not to bear arms unless properly exchanged. Then you were sent back South, where you went to a camp for parolees. You were still in the army but could not rejoin your unit until exchanged. At no time were you required to take the oath, altho you could if you chose and remain the North.

Every couple of months representatives from both sides got together and "exchanged" prisoners. This just meant that a certain number of parolees from each side would be released from the parole camps (which were, remember in their own territory) to return to their units. The numbers varied -- a general was worth so many privates, for example.

The system broke down in mid-1863 and paroles pretty much ceased. This is what led to the horror of the prison camps -- neither side had ever contemplated actually holding large numbers of prisoners on a more or less permanent basis. The camps early in the war were really only transit facilities.

After that, however, all Southern prisoners were required to take the oath to be released.

Hope this helps.

Fred