Proposed Canon
A Unit assigned to the Garrison of a City is given the Garrison ability. The Unit's Move is set to 0 and it cannot leave their assigned City's zones. The Garrison Unit can freely move throughout the City both on and off Turn without expending Move. A Unit can later be re-assigned to the field, at which time its Move is restored to its normal value.
 Known Garrison Units
Assignment to Garrison and re-assignment to the field likely takes place at night, or off-Turn. It is unlikely a Unit would be allowed to expend Move in the field, end Move in a city, and then expend unlimited Move after assignment to Garrison all in one Turn. This has potential exploits.
Garrison units may have some advantage to offset their lack of Move, perhaps reduced upkeep costs or increased effectiveness within their assigned city.
When the armies of Gobwin Knob were closing in on her capital, Queen Bea of Unaroyal decided her side had no hope of victory and she chose to deny GK further decrypted units. In her letter to Don King she claimed to have exhausted her treasury promoting all of her garrison units to field units. Then she ordered all units outside of the city and walked herself through the magic kingdom portal which disbanded all of those field units.summer-updates-043
Parson's Klog 12 said if a ruler without heir gets croaked, 'field units dispand'. But it also says 'everyone here' is frozen. I guess that 'field unit' there only means unit in the field in the sense of outside the city. And not the opposite of garrison unit.