In ISDN it is a field in the header of the D- channel protocol of the network layer. The message type defines the function of the message. To establish an ISDN connection, the connection setup must be initialized, followed by compatibility confirmation functions, call acceptance by the destination terminal, and acknowledgement. When the connection is terminated, the termination function is initialized and the message is acknowledged accordingly.
In ATM, the Message Type is the field containing the bit flags of the RM cells.