[Marcello]

It is quite simple.
There four weapon slots, therefore a max of four distinct weapons. The first weapon slot has four ammunition slots, HE/AP/Sabot/HEAT.
The other three weapon slots have only HE/AP.
The names don't have to be taken too much literally: Sabot can represent both arrowhead and actual discarding sabots rounds or simply different sabot types depending on the weapon stats.
And via a software trickery you can have AP rounds in the last three slots representing HEAT wrheads.
The max number of rounds that an ammo slot will hold is 255.
There is other stuff, such as being able to fire SAM at ground units etc. but the above is most of it.

It is somewhat limited, in that you have to fudge to represent non standard ammunition such as flechette or HESH, barrel fired ATGMs have to be treated as an extra weapon etc. but it has always been that way. It is sufficient to represent most of the tanks most of the time but nothing more, I suppose it was done for sake of memory/CPU/programming/budget/whatever back in the days.