There is a number of communication protocols that can be used in conjunction with the SpaceWire Standard (ECSS-E-ST-50-12), to provide a comprehensive set of services for onboard user applications. These protocols are covered by the ECSS-E-ST-50-5x series. To distinguish between the various protocols a protocol identifier is used. This Standard specifies this protocol identifier. This standard may be tailored for the specific characteristic and constrains of a space project in conformance with ECSS-S-ST-00.