Recommendation ITU-T Z.104 defines the data features of the Specification and Description Language so that data definitions and expressions are well defined. Together with Recommendations ITU-T Z.100, ITU-T Z.101, ITU-T Z.102, ITU-T Z.103, ITU-T Z.105, ITU-T Z.106 and ITU-T Z.107, this Recommendation is part of a reference manual for the language. The language defined in this Recommendation partially overlaps features of the language included in Basic SDL-2010 in Recommendation ITU-T Z.101 and used in Comprehensive SDL-2010 in Recommendation ITU-T Z.102 and the features of Recommendation ITU-T Z.103.