According to Apple, releasing devices is applicable to devices will no longer in DEP. For example, device lost, permanently damaged, or students graduates or leave schools for new study life.
After released, if the devices need to be added into DEP again, then this can be done via Apple Configurator 2.
Reference：Release devices in Apple School Manager
Whereas unassign keeps the devices in DEP, just simply remove the devices from ESP MDM server. In this case, the new MDM solution provider can take over the management by assigning to the new one in ASM.