The present document establishes a set of guidelines for the formal use of Specification and Description Language (SDL) for descriptive@ rather than detailed design@ purposes. The objective of the guidelines is to provide assistance to rapporteurs of behaviour standards so that the SDL that appears in ETSI deliverables is formally expressed@ easy to read and understand and at a level of detail consistent with other standards. The present document applies to all standards that make use of SDL to specify protocols@ services or any other type of behaviour. Users of the present document are assumed to have a working knowledge of SDL. The present document should not be considered to be an SDL tutorial and should be read in conjunction with EG 201 383[ 1]@ ETR 298 [2] and EG 201 015 [3].