Unit bitmap types?

[Crossroads]

Can anyone refresh my mind again what the various unit bitmaps are for?

First, the files on the main folder

Up0000.bmp (basic unit picture / "mid" war unit picture?)
Mp0000.bmp (unit picture when loaded?)
Xp0000.bmp (When on water? And when on snow?) Is this the reason some of the Finnish ski capable units are on skis when on shallow water or pontoon bridge although on a summer scenario?

Then, the Specials folder

E-Up etc ("early" war unit picture?)
L-Up etc ("late" war unit picture?)
S-Up etc (Winter?)

And after that, do all the countries support this functionality? Say if I provide E-Up, L-Up and S-Up bitmaps for say Axis Soviet Union (Country code 24 in EF), does it work?

And are there differences between EF, WF and RS regarding this?

Lots of questions I know