An unit deserts if:
A deserter does not belong to any armed or section.
Any unit other than a commander can shoot on a deserter (see
Attack of deserters), but it will not gain any xp. It will lose on the other hand AP when attacking.
A deserter has 100% of tiredness. As soon as it is engaged by a section (see
Engaging units), it will then be necessary to wait in order to have AP enough to rest it.
The interest of a deserter is that one can take it to supplement at low cost his section.