There are some achievements for “killing” or not killing units. What is the difference between “downing” a unit and “killing” it?
You will notice that typically when you defeat an enemy unit, or have a unit defeated, they remain on the battlefield for a short period of time while a helicopter evacuates them. This unit is “downed” and if they are evacuated successfully, they will be available for use in future battles and retain their rank and any XP earned in that battle will be credited to them. If you command your units to continue firing on this unit, it can be killed entirely. When units are killed they lose their rank completely and are replaced by a fresh recruit unit.