Indirect Combat is combat initiated when an indirect combat unit attacks another unit. Unlike Direct Combat, when combat is initiated with at least one combatant being an indirect unit, the defending unit can't attack. This means that if an indirect unit attacks, the defending unit can't counterattack and vice versa.
Besides the Days of Ruin Anti-Tank, all indirect units have a minimum range that prevents them from attacking nearby enemy units. Because of this, retreating indirect units is hopeless when they've already been engaged by the enemy, so good positioning must be maintained in order to prolong constant indirect combat.
Indirect combat units are often useful near chokepoints, and they are deadly in Fog of War where it is very hard to spot an indirect combat unit hiding in Woods or behind Mountains, allowing them to bombard any army continuously and freely.