They fill what they need at the time. Service recovery and dispatchers are all “GOC Specialists” and have the same base pay. (The overrides are a little different). Service recovery is more of what you might consider “flight coordinators” at other airlines.
Monitoring delays, swapping a/c, setting up adhoc flights, etc etc.
But FedEx requires all GOC specialists to have a dispatch license. You can move from service recovery into dispatch if you desire, usually after a year in the position if there’s a “flight” opening. Some people get in the Service Recovery side and decide they like it so they stick with it. Service Recovery people still have to go through dispatch OJT and a desk check albeit a little more accelerated training. Service Recovery still has to do an annual FAM ride like the dispatchers.