The creature calculates either a simple parabolic trajectory or a firing solution for indirect fire, then lets go. These types of attacks do not require direct Line of Sight, but are very inaccurate or impossible without an allied unit that can Spot (3 AP) targets.
Indirect Fire hit rolls are always INT-based.
Not all weapons can perform Indirect Fire, while some have an easier time than others performing it. The following weapon types are capable of Indirect Fire:
CAN perform Indirect Fire with NO penalties, at their stated RNG.
CAN perform Indirect Fire, adding a Hard DM, and only at their stated RNG.
CAN perform Indirect Fire, adding a Hard DM, across an entire Tactical Map.
CAN perform Indirect Fire, adding a Very Hard DM, across an entire Tactical Map.