This Recommendation defines the data and action language features of the Specification and Description Language. The features defined in this document include: encoding and decoding of data communicated over channels, the definition of data types (including interfaces), the definition of variables (including parameters), the definition and application of operations, the evaluation of expressions and assignment of values to variables. Without these features the language would be of limited use. For this reason, even very simple specifications in the Specification and Description Language use ITU-T Z.104 features. Together with [ITU-T Z.100], [ITU-T Z.101], [ITU-T Z.102], [ITU-T Z.103], [ITU-T Z.105] and [ITU-T Z.106], this Recommendation forms a reference manual for the language.