As I understand it: (and correct me if I am wrong)
You can have your units recover when they are either Garision or Active status. With no effect on healing time wether the status is one or the other.
The DDC is lower to recover a unit in Garision status.
The DDC is higher to recover a unit in Active status, but sometimes the situation might require that units are in Active status to be able to recover.
When recovering the unit does nothing else.