I just had to go into the code and make sure I wasn't telling stories. I have a note to fix an issue with this in my own game, but when you said that it made me wonder if it's something I actually made a plugin for and just didn't remember.
So I can confirm the following:
If you specify a battleback on the map properties, or use a Change Battle Background command, that's the battleback that will be used.
If you do neither of those, the battleback will depend on the "type" assigned to the tileset.
If it's "world" type, each ground tile is associated with a combination of the RTP backgrounds (grassland, desert, swamp, etc), and the battleback is built from 2 images, depending on what tile you're standing on when the battle is initiated.
If it's "area" type, there's no such association between the tiles and the battlebacks. It just takes a screenshot of the map (not the characters), blurs it, and uses that.
The issue I have to fix in my game is that if there's a character speaking and they have a speech bubble, the bubble is included in the battleback, which is not desirable. I'm pretty sure the character is not included, so maybe events in general aren't included. This makes me wonder about events which are part of the landscape - doors, for example, you'd want to include. And they may be events with tiles, or events with character sprites. One day I'll go take a look and see if I can figure out the logic.