The Aid Station is a special building that is not constructed by Engineers, but constructed by Medics with veteran rank, anywhere on the battlefield. It can also be built by a squad of veteran S3-55 Wards.
If a squad of medics build an Aid Station, they will be consumed and you no longer have that squad. But if a squad of S3-55 Wards builds an Aid Station, they will not be consumed.
It can be a handy reinforcement spot for damaged infantry squads, useful for establishing a forward base to support infantry-based combat.