Portus said:
This is essencial to understand the next step, unit and art XMLs, and my experiments are unconclusivf
Well you see that's the thing, I think you should first understand the XML and just epxeriment adding new units using the existing graphics, and then move on to adding new unit graphics.
Still, let me address the specific questions you posted:
What are all that files mean?
NIF - this is the file that contains the model itself and references the textures. Both the regular and the xx_FX version are needed in most cases for the unit to properly work.
DDS - these are the texture files used by the model, i.e. the NIF file.
KFM - this file contains information about the animations that the unit/model has.
KF - these are the actual animations.
If you simply want some unit to have a different skin you can just modify the appropriate texture file (for example swordsman_128.dds for the swordsman unit) and put it in the appropriate folder in customassets (or if it's a mod it'll be assets folder), in this case it'll be art/units/swordsman. If you want, for example, to have two swordsmen using different skins, then you'll need to copy the swordsman NIF files together with the new texture into a different folder and reference that in the artdefines_unit file. Something like:
Code:
<NIF>Art/Units/Swordsman_2/Swordsman.nif</NIF>
<KFM>Art/Units/Swordsman/Swordsman.kfm</KFM>
<SHADERNIF>Art/Units/Swordsman_2/Swordsman_FX.nif</SHADERNIF>
Unless you're deep into unit modding you'll never need to copy the animation (KF, KFM) files, you can always refer to the original ones, or even different ones. Some units can use animations from other units,
check out this thread for more detail. You also don't ever need to rename the NIF files themselves except to simply change their name, but you can freely do that, i.e. it won't ruin the file in any way except you'll of course need to change the entries in the XML files. Also, changing the name of the NIF files will not change what texture they use. I.e. swordsman.nif renamed to anything will still use the swordsman_128.dds as texture.
What the relation betwen the main folder name and that filesnames?
Absolutely none, as long as all the relevant XML entries have been changed you can change the names of the NIF files or the folders to anything. You cannot however change the texture names or the animation names.
what files can i rename?
Already answered that one. In general however, if you simply use the existing models or models you download for new units you shouldn't have any reason to rename any files.
Why most of new units have only 2, max 5 files only?
I assume you're referring to new units that people make. Most of these units don't have any new animations they simply use animations from existing units. So all they need to provide are the nif files and the textures. In these cases the author either specifies what animations the unit uses, or it's clear from the name of the file. For example some of my units that are just modifications of existing ones use the old unit's name, i.e. a warrior with a different weapon would still have the filenames warrior.nif and warrior_fx.nif, so it's obvious it should use warrior animation. Of course it does need to go into a different folder than the vanilla warrior. On the other hand snafusmith's or nautil's units (the tanks and the planes) usually have their own names but use animations from vanilla tank or fighter or jet fighter. In such cases the artdefines entry (part of it) should look something like this:
Code:
<NIF>Art/Units/nautilsplane/nautilsplane.nif</NIF>
<KFM>Art/Units/fighter/fighter.kfm</KFM>
<SHADERNIF>Art/Units/nautilsplane/nautilsplane.nif</SHADERNIF>
As you can see it doesn't have an _FX version, that means it doesn't support team colors. Some new units will come like that, others however will support team colors and in that case they'll have _FX version.
Finally there are some units that are complete and have all the files (including animations), in this case you just set them up like any other unit.
Hope this helps
P.S. You should've posted this in the main Creation forum and gave it a better title.
tanks 
