Halliday & Matthiessen (2014: 289):
These two types also come in two modes of being, ‘attributive’ and ‘identifying’. We can thus recognise the following series of proportions:
intensive Emily is a poet : attributive
Emily is the poet :: identifying
possessive Emily has a piano : attributive
the piano is Emily’s :: identifying
circumstantial the meeting is on Friday : attributive
the time of the meeting is Friday identifying