The label provides some kind of a definition of the units that have been identified as parts of some larger whole. There are in principle two significant ways of labelling a linguistic unit. One is to assign it to a class; the other is to assign a function to it. Hence there are two principles according to which we can label the constituents of a grammatical structure: (i) by CLASS, and (ii) by FUNCTION.
- Halliday (1985), Pag. 27