What are HEAT and SABOT rounds that are available in OFP tank models?

In one simple sentence, use Sabot rounds against armored vehicles, like APCs and tanks and use HEAT rounds against infantry, cars, trucks and buildings. For a detailed explanation of the difference between Sabot and HEAT rounds, read the following:

Tank projectiles are basically broken down into two types: shaped charge rounds and kinetic-energy penetrators. The two rounds differ in both composition and concept. Shaped round charges--or HEAT charges--focus the energy from an explosion into a small, concentrated area in order to penetrate armor plates. In order to defend against such projectiles, scientists developed combination armor that combined the hardness of steel with the fluid properties of ceramics. Combination armor combines a layer of honeycombed ceramic sandwiched by two layers of steel plates. Although the HEAT round easily penetrates the outer steel plate, the ceramics "flow" around the jet and break the jet into smaller components, spreading the force of the blast out into a larger area and reducing the effectiveness of the HEAT round.

The innovation of combination armor required the use of kinetic energy to punch through armor. This was no new inspiration, but was the original concept behind tank projectiles. Instead of relying on the HEAT round's explosive jet to pierce armor, kinetic-energy penetrators rely on mass and velocity. The armor-piercing fin-stabilized discarding-sabot (APFSDS) rounds contain very dense, long, slender darts, and are called long-rod penetrators. Commonly referred to as sabot rounds, these rounds burrow into a tank's armor upon impact. If the round contains enough kinetic energy, it will pass through the tank's armor and destroys whatever is inside. Often times, however, the round does not contain enough energy to penetrate all the way through. Although the round does not fully penetrate the armor, it creates many small armor fragments that discharge into the tank, causing destruction inside.

With rounds that use both kinetic-energy and the concentration of explosions to penetrate armor, scientists constantly research new ways to defend against such rounds and improve the survivability of armored vehicles. Using a combination of armor thickness, innovative materials, (combination armor and depleted uranium, for example) and armor slope assists the scientist in protecting against the different types of projectiles. The latest type of protection comes from reactive armor. Reactive armor is described as a sandwich of explosive between metal plates that explodes when a round strikes it. The explosive in the reactive armor detonates at the same time as a HEAT round, throwing the steel plates out against the round and highly disrupting the jet. Reactive armor's effectiveness against long-rod penetrators, however, is less significant because the sabot round is too massive for the steel plates to block.

Additional descriptive and technical information about HEAT and Sabot rounds can be found on the following pages at FAS.ORG: