Criticals means special actions taken by the unit, in infantry case:
- They could die with one hit and still have plenty of health.
- They could die without being able to be picked up by medics.
- They could "die" and picked up by medics.
- They can be dismembered IIRC.
in vehicles:
- Kill Gunner/Commander.
- Disable tracks
- Disable engine.
- Disable main gun.
The criticals are measured by probabilities and each critical has one assigned. There are three levels of criticals, when you have the health replenished, when not and when your unit has very low health (where used to occur the 5% bug on tanks).
The criticals can occur every time a proyectile hits an enemy unit.