Chaff is an colloquial term for inexpensive and disposable units. In battle, they're often of very limited value, but they might be used to delay the fight, overwhelm the opponent, or bait mages into wasting casts of evocation spells to deal with them. Because they're usually numerous and easy to mass, they can also be good candidates to buff.

Also often mentioned is "siege chaff", units with otherwise low value but remarkably high siege strength, that can help breaking down enemy forts. In the same vein, "patrol chaff" means units with good patrol abilities that can bring down province unrest or track down pesky spies.