Retreat zones were first introduced in Zero-K as an area that units would automatically retreat to once their retreat condition was met. Units would be configured to retreat if their hitpoints were reduced to 30, 60, or 90%.
The retreat zone is a circular area designated on the ground that a unit will retreat to if it is sufficiently damaged. The idea is to have engineers patrolling to area to repair anything nearby.
Units will retreat to the zone and then continue carrying out their previous orders once repaired completely.