This is the Codex Proprietary Protocol ID value placed in the first byte of the Protocol Identifier field within the Call User Data of Call Request packets, used for protocols that haven't been assigned a standardized value by the CCITT. An admin only changes this when integrating with equipment from another vendor or era that expects a specific proprietary protocol ID in that field; getting it wrong could cause call setup to be rejected or misinterpreted by devices that inspect this field to determine how to handle the call's payload. For example, interconnecting this network with a different vendor's legacy PAD equipment that expects a specific ID in that byte would require setting this value to match what that equipment recognizes.