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 roles: Identifier and Identified, and Token and Value. These latter intersect with each other, so there are two possible role combinations:(i) Identified/Token and Identifier/Value ['decoding'];(ii) Identifier/Token and Identified/Value ['encoding'].