Men who were on active duty in the field, and who were surrendered by the various army commanders over time through April and May, were paroled at the time they laid down their arms, and were allowed to go home. They were, however, still considered Confederate soldiers. By signing their paroles, they promised not to take up arms against the US unless duly exchanged.
The majority of prisoners at Rock Island, and at other POW camps in the north as well, were not released from prison until after the issuance of General Order #109, dated June 6. Of course, it took some time afterward to get all prisoners processed and release. These men were not paroled, but released based on taking an oath of allegiance to the US. That oath, although it did not state this in so many words, was a tacit recognition that they were no longer soldiers. Note that they all had to take the oath of allegiance, but afterward were to be afforded the opportunity to take President Johnson's amnesty oath.