The Service Information Octet field in a MSU (Message Signalling Unit) contains the 4bit subservice field followed by the 4-bit service indicator. The subservice field contains the network indicator (e.g. national or international) and the message priority (0 to 3 with 3 being the highest priority). Message priority is considered only under congestion conditions not to control the order in which messages are transmitted. Low priority messages may be discarded during periods of congestion. The service indicator specifies the MTP (Message Transfer Part) user thereby allowing the decoding of the information contained in the payload.