The tooltip on the maps says that when you don't specify a battle back, if the map is a "world map", one is automatically selected for you based upon the terrain tile the player is on. For all other maps, the battle back is simply a transparent overlay of the map itself, unless you specify a battle back.
How it determines the difference between a "world map" or any other regular map, I have no idea. I suspect it has something to do with the tilesets and whether or not you specify as "world type" or "area type" under the "mode" in the Tilesets. Though, this also affects how the Autotiles work as well... I also don't know of any way to determine which tiles conform to which battlebacks in the "automatically selected" feature. The "Help File" is... well... less than "helpful" on the subject.
There's probably a way to do it using "Terrain Tag", but you're only given numbers 0 through 7 to work with, so I'd wager it probably wouldn't work very well.
Ah, how I miss the good old days of simply setting which tiles were which terrain in the maker... I wish I knew why features from previous versions of the makers were abandoned that were actually very good "quality of life" features, in favor of a nebulous and silly "just program it yourself!" mentality of adding plugins to do basic things that previous makers used to be able to handle by default.
I actually wonder if there's a way to specify battleback by "Region ID". If there's not... it might be worth requesting it as a feature in future RM Engines... or an update to the existing MV... The way the Region ID thing works would actually make it ideal for selecting Battlebacks by location, since you can select troops by location...
