I expect to have a first internal test version of the Skinner's Pack in a week or two, so things will then become clearer and you can see things working, which is often easier than explaining them, especially in a foreign language. If you have BoB2 and and want to test it, Adlabs, you will be welcome

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So, here only in telegram style:
The idea is to have a VERY flexible system that allows the skinner (without help from a modeller or someone else) to either change the "baseskin" according to almost arbitrary criteria, for example change from one scheme to another for different units at different times.
Using the same criteria you can choose "decals" and can determine where to put them and how large they are. For example you can say "unitA from 17.11.1940 onwards has as emblem xyz.bmp in large on the cowling, but the lead plane instead has a inscritption in extra large under the canopy". Or "the decal appears when the gear is down and this when it is up". So, you could light up the gear indicator. Or say the modeller of the Me110 decided to use the me109 tires since they in his opinion are the same and you disagree, you can say "if it is a me110, replace the tire by texture MaSupaTires.bmp".
I once did something similar in the furniture industry for door handles and people used it for stuff way outside of what was originally envisioned.
To be honest I have no idea how say Il2 does it. However, I know SF:P1 has 4 criteria to go by (nation ID unit ID plane ID and a 4th I forgot). I plan to have WAY in excess of 100.
To answer directly, you can certainly use it for ID numbers, letters, emblems etc. You can also use it for roundels it it does make sense if there are several styles like the larger more centred ones and the smaller ones placed nearer the wing tips. If a base skin like say ther black/white underside one always uses one style of roundels, there is no reason to use Multiskins for this, just "bake" them into the baseskin.