Even if you don't need it now, it's best to learn how to deal with such problems now, when you first encounter them. You're bound to need to in the near future if you're really serious about your mod.
When spritesheets are set for a particular map, the game reads the frame rects of the entities on that particular map from the set spritesheet. It only reads the areas in which the sprites are SUPPOSED to be at. If you set the wrong spritesheet, the game reads the areas on the spritesheet set for the map wherein the entities' sprites are supposed to be.
For example, Bats.
If you put a First Cave bat on a map, but set NpcRegu.pbm as the spritesheet to be read from for that map, your bat's gonna look like a hovering flickering compilation of sections from Santa's body.
Edit -
Btw, there are 2 spritesheet slots per map. However, it seems like you don't know what each box's purposes are.
Well, here they are
:
Box #1 (Basically all mob spritesheets)
======
Cemet
Cent
Eggs1
Eggs2
Guest
Hell
Island
Kings
Maze
Moon
Plant
Priest
Ravil
Red
Sand
Stream
Toro
Weed
Box #2 (Basically most boss spritesheets)
======
Almo1
Almo2
Ballos
Bllg
Curly
Dark
Dr
Frog
Heri
IronH
Miza
Omg
Press
Ravil
TwinD, X
OH AND this applies to your very own self-created/-edited spritesheets. Non name-sensitive.