Halliday & Matthiessen (2014: 262):
The configuration of Process + ‘Be-er1’ + ‘Be-er2’ opens up the potential for construing the abstract relationships of class–membership and identity in all domains of experience. Class–membership is construed by attributive clauses and identity by identifying ones. These two ‘relational’ clause types cut across the inner and outer experience of ‘mental’ and ‘material’ clauses …