Mix and Match is a feature that lists segments of the workout on a menu, then you choose one of the segments, it plays, then the program goes back to the mix and match menu so you can choose another segment if you want.
Chapters are found on many workouts (and in movies), but mix and match is a less common feature.
There's also a "workout blender" feature in the Body Blast series. For that, you choose up to 5 (I think!) segments on a menu, then hit play, and the segments play in order (unlike the "mix and match" feature, the "blender" feature doesn't go back to the menu after each segment, so creates a smoother workout, but limits the number of segments you can string together---unless you do it several times in a row!).
Isn't it great to have well-chaptered (well, at least pre-Hardcore) DVD's? I have several Goodtimes workouts (Firm, Tae Bo), and they are all unchaptered. The main screne just has "play" as a choice. Very annoying!!