Halliday & Matthiessen (2014: 461):
In elaboration, one clause elaborates on the meaning of another by further specifying or describing it; … The secondary clause does not introduce a new element into the picture but rather provides a further characterisation of one that is already there, restating it, clarifying it, refining it, or adding a descriptive attribute or comment. … The thing that is elaborated may be the primary clause as a whole, or it may be just some part of it — one or more of its constituents.