Reissue is kind of an umbrella term for remaster and repack
When a film is repacked (like the new mission impossible sets) they take all of the original discs and just replace the old artwork with the newly printed art, re-shrink wrap it and send them back out
I picked up M:I and M:I:II recently and they had some of the original shrink wrap still left under the new shrink wrap
If the studio is willing to spend the money to remaster a title to improve upon the initial release, they will sometimes include new extras, but not always (for example, the remastered Fifth Element and silent remaster of The Fugitive don't have any new extras)