opal spire
If a unit starts shooting at something when set to fire at will, then you order them to hold fire, they'll keep shooting for quite a while before stopping - much slower than if you issue a stop command after setting hold fire.
I tested this with pawns, thugs and grunts.
strong harness
Hmmm.
Probably something to do with how often the engine auto-acquires targets for units.
Last I checked, units with a current target only "search" for better targets every 5 seconds or so.
So I guess hold fire does not immediately clear a unit's target, and needs to wait for that update