From ErfWiki

Proposed Canon

Instead of croaking an enemy, a Unit can attempt to capture it. A Unit that is captured is in the possession of the capturing Side.

When a city falls, all remaining garrison units are automatically captured, with shackles popping onto them.

A captured Unit takes Turn with the Side that captured it.^WoT-2786759 A Unit's upkeep is paid by the capturing Side, and so the food tends to be lower quality.

Speculation

Capturing for intent of forcing a Unit to change Sides is generally reserved for Casters, who are rare and valuable, and not Warlords, whose duty and loyalty make them unlikely to join their captors. Warlords and basic troops may be captured for interrogation purposes, ransom, or some other reason.

Known Captives

• Jillian has been captured by Gobwin Knob forces many times, only to "escape" each time.
• Charlie offered to capture Parson (offering him employment with Charlescomm) and later made capturing Parson a requirement for Prince Ansom to retain Charlie's services, with penalties for croaking him.
• When Stanley destroyed Faq, the Foolamancer Jack Snipe was captured and changed his loyalty to Stanley.
• Wrigley was a Unaroyal piker captured and executed (and presumably decrypted) by Gobwin Knob.