Halliday & Matthiessen (2014: 292):
In the ‘identifying’ mode, the circumstance takes the form of a relationship between two entities; one entity being related to another by a feature of time or place or manner etc. As with the circumstantial attributive, this pattern may be organised semantically in one of two ways. The relationship is expressed either
(a) a feature of the participants, as in tomorrow is the tenth, or
(b) as a feature of the process, as in the fair takes up the whole day.