For loading of battlebacks it goes like this (based on the Spriteset_Battle):
Check if it's battletest -> If yes, load background from troop settings in the database
If not
Check if there is a specified battleback -> If yes, use that
if not
Check if it's an overworld map -> If yes, it loads a background based on the autotile (Haven't tested this one) *
If not
Loads a blurred background based from Scene_Manager something (from the description, it seems to be a super blurred image of the map that you're in)
as for tile association to battlebacks, well I and Yanfly have scripts that somehow does that... though instead of a specific tile, both of our scripts rely on map regions... the only difference with our scripts are the set-up part (yanfly does the set-up using map notes, I do the set-up using the script itself)
* When it's oveworld, it checks first if the player is in a vehicle and loads up RTP backgrounds based on that... Else it loads battleback based on the autotile (maybe where the player is), and loads the battleback from RTP backgrounds based on the autotile ID, else it will load the default background which is the grasslands battleback...
from the looks of it, overworld maps are maps that uses tilesets set as Field Type...
based on this, I think it shows that if you aren't using the whole RTP, not defining battlebacks for your maps might cause problems because you might trigger one of these overworld scenarios and not have the required file...
