Purpose of support
You can send your troops to your own or an alliance member's settlement. After arrival, they remain there until recalled and defend alongside its local garrison, using that settlement's wall, Unity Monument and other defence bonuses.
Unit ownership
Support troops always belong to their settlement of origin and continue occupying population capacity there, even when supporting another settlement of the same player. They must return home before they can be sent elsewhere.
Losses and recall
When support troops die, their occupied population capacity is released at the origin. Their owner may recall selected units, while the receiving player may send an entire foreign support force home. There is no limit to the number of support troops stationed in one settlement.