Lets understand via example
ERAB Release
Messages: ERAB Release Command and ERAB Release Response
Now the message type is there for ERAB Release Command and ERAB Release Response.
Now message type contains procedure code and and type of message (initiating, successful/unsuccessful etc)
Procedure code for ERAB-Release
id-E-RabRelease : 7 (refer page 221 136.413 ver 10)
Hope it would be clear now...