1) Class A GPRS MS
Class A GPRS MS can simultaneously be connected to both GSM and GPRS networks and activated in both networks, monitor information of each system and start them simultaneously, and provide GPRS service and GSM circuit-switched service simultaneously, including short message service. Class A MS subscriber can initiate/receive call in both services and handover automatically between packet data service and circuit service.
2) Class B GPRS MS
Class B GPRS MS can be connected to both GSM and GPRS networks simultaneously. It can be used in GPRS packet service and GSM circuit-switched service separately but not simultaneously, that is, in a certain moment, it uses either circuit-switched service or packet-switched service. Class B MS also can automatically handover.
3) Class C GPRS MS
At a certain moment, Class C MS can be connected only GSM network or GPRS If it supports both packet-switched and circuit-switched services, the service handover should be completed manually. It cannot use both services simultaneously
source: overview GSM & GPRS
