Precedence is the priority associated with a call.

A precedence call is a call with precedence level higher than the lowest level of precedence.

MLPP Service Domain: An MLPP service domain consists of a set of MLPP subscribers (MLPP users) and the network and access resources that are in use by that set of MLPP subscribers at any given time. Connections and resources that are in use by MLPP subscribers may only be preempted by higher precedence calls from MLPP subscribers within the same domain. The Defense Switch Network (DSN) service domain is zero (0). Stations and trunks may have their own service domain and preemption can only occur in the same service domain.

Network: In this standard, "network" refers to all telecommunications equipment that has any part in processing a call or a supplementary service for the user referred to. It may include local exchanges, transit exchanges, and NT2s but does not include the ISDN terminal and is not limited to the "public" network or any other particular set of equipment.

MLPP Call: An MLPP call is a call that has a precedence level established and is ether being setup or is setup. In Digital Subscriber Signaling System number 1 (DSS1: ISDN Q.931 signaling), an MLPP call is a call from an MLPP subscriber for which a setup has been sent but no DISCONNECT has been sent or received.

Preemptable Circuit: A preemptable circuit is a circuit that is active with or reserved for an MLPP call: (a) within the same domain as the preempting call and (b) with a lower precedence than the preempting call. A busy or reserved circuit for which a precedence level has not been specified is not a preemptable circuit.

Preemption initiating exchange: A preemption initiating exchange is the exchange that is congested (i.e. no idle circuits) and has received a preempting call setup.