Halliday & Matthiessen (2014: 276):
In the ‘identifying’ mode, some thing has an identity assigned to it. What this means is that one entity is being used to identify another: ‘x is identified by a’, or ‘a serves to define the identity of x’. Structurally we label the x–element, that which is to be identified, as the Identified, and the a–element, that which serves as identity, as the Identifier.