"HEAP" doesn't exist... you can have:
AP - Armor Piercing ("shot"). Also used as a generic term, unfortunately.
APHE - AP with a High Explosive "bursting charge". Penetrates through standard AP mechanics; bursting charge may explode behind the armor, if the projectile is still intact.
APBC - AP with a ballistic cap. Unhelpfully, may also mean APHE with BC. Intended to improve the exterior ballistics of the projectile.
APC - AP, Capped. A softer steel cap on the end of an AP (or APHE) projectile, to help with handling sloped armor plate.
APCBC - Armor Piercing, Capped, Ballistic Capped. Combine APC and APBC.
APCR - AP, Composite Rigid - usually tungsten-cored projectiles.
APDS - AP, Discarding Sabot - sub-caliber AP munition, with a sabot (jacket) that allows the gun to send a smaller, denser projectile down range at high velocity
APFSDS - AP, Fin-Stabilised, Discarding Sabot - the advent of long-rod penetrators.
HEAT - High Explosive, Anti-Tank - originally HE rounds, become shaped charge weapons.
HESH - HE, Squash-Head - another variant on HEAT.
And, having mentioned "exterior ballistics", consider the following terms:
Interior Ballistics - the performance of the round and projectile in the gun tube, from the moment of firing, until the projectile exits the barrel. NOT modeled in RO2.
Exterior Ballistics - the performance of the projectile from the moment it leaves the barrel until it impacts something. Modeled in RO2.
Terminal Ballistics - what happens from the moment the projectile impacts something (i.e. armor plate) until it is destroyed or penetrates the armor. Modeled in RO2.
Behind-Armor Performance - what happens if a round penetrates armor plate in any form. Modeled in RO2.