Concerning the SDRs, in order to formally define a relation the following four fields ought to be defined (see also<cite> [5]</cite> ): 1. The relation's type (i.e. Synchronic or Diachronic). 2. The relation's name. 3. The set of pairs of message types that are involved in the relation. 4. The constraints that the corresponding arguments of each of the pairs of message types should have.