Hugh Simmons of the Fort Delaware Society has created a very helpful chart showing dates and events involving release of Confederate prisoners in 1865 --
http://www.fortdelaware.org/going%20home%20-%20the%20war%20is%20over.htm
"Why does is say his release"?
Release is what happens when a prisoner is allowed to leave. I don't know if you needed a further definition or not, but that's what 'release' means.