The preference of the Protocol in the MGC group.
The lower the number the higher the preference. i.e. 1 will
have higher preference than 2.
When gateway does not know which protocol is used by a call
agent the preference is used to make the decision.
But if gateway knows which protocol used by a call agent,
gateway will use that protocol (as long as the protocol is
added to the MGC group, irrespective of the preference) to
communicate with the call agent.
For example:
when gateway return a call agent message, gateway
will use the protocol which used by the call agent to
send the message.