Based on Googling, it appears that rehab units provide "rehabilitation" support to first responders (rehydration, rest, cooling/warming). There seems to be quite a variety. Some agencies have rehab units that are staffed by volunteers, others have a regional shared rehab vehicle. Sometimes it is a bus but often a retired ambulance gets repurposed for rehab. Depending on the climate they are outfitted with AC and/or heating. Some carry misting units to set up a "cool" area to which personnel can retreat. As you mention, they are generally only rolled out to large or extended incidents.
P.S., I've been happy with my 396T