Halliday & Matthiessen (1999: 117):
elaboration is a (partial) identity relation between figures: one is identified with another with a difference in perspective (it matters a lot; it plays an important rôle) or one is included under another as an example (it plays an important rôle; e.g., it provides the infrastructure). These are clearly related to one another: identity is the limiting case of inclusion and inclusion is partial identity.