@krullapa As I said, it's an actual recording of all actions performed during a game; constructions started / stopped / cancelled, units commanded to move or attack or whatever else, buildings placed and sold, all that stuff. From this data and the map it can replay the game in the game engine exactly like it was at the time these actions were recorded.
But the game only keeps that recording from the last played skirmish, and since this is actually a feature meant for multiplayer games, it can't deal with saving and loading, and is unable to automatically knit together pieces of your gameplay to one whole if you start saving and reloading stuff. It'll simply delete the last one and start over at the moment you load a game, counting it as a new game.