Halliday & Matthiessen (1999: 151-2):
In the limiting case, there is only one participant, the Existent; but generally there are two participants, the one being related by the process to the other. They may be being related by ascription, as Attribute to Carrier; or by identification, a rather complex relationship involving two pairs of participant rôles: Identifier and Identified, and Token and Value. These latter intersect with each other, so that there are two possible rôle combinations: (i) Identified/Token and Identifier/Value [decoding]; (ii) Identifier/Token and Identified/Value [encoding].