Slitherine

What is the meaning of "Experience cap has been set to 325"?

It means none of your units can get more experience then 325. After you reach 325 experience on any unit all further experience this unit would gain simply disappear. There are multiple ways to deal with it discussed in depth on this forums. Just search for it to read up the details but it's basically narrow down to the three following ways:
a) accept the XP loss and just keep using your unit like usual
b) rotate said unit to reserve and start gaining experience for a new unit so you have some experience for it when additional core slots opens
c) start using green reinforcements for said unit to save prestige. This will of course reduce the XP again but you should be able to regain some on the next scenarios.

OK, understood. But for translation, is that the term could be used: "Maximum Capacity of Experience"?

Why ? The meaning of cap (upper boundary) seems clear enough ?

Id rather is read, 3 and a quarter stars as cap. Everyone seems to know that stars are exp, but not everyone knows each star is 100 etc.

Gentlemen, for only a matter of translation, I'm trying to make a complete translation for Brazil and sometimes abbreviated names and acronyms can not be understood by all.

Its not really a max, as units can have more XP than this. But it means that you can't gain any more xp once your unit hits 325 (or whatever the cap is).

So for example, in West 45, your xp cap is reduced below the west 44 cap. So your elite units may actually have more xp than 325, but they can't gain any more xp until their xp drops below 325.

Thanks Endurance, understood the concept.