Halliday & Matthiessen (2004: 230):
… either the Token is ‘decoded’
or else the Value is ‘encoded’.
If the Token is construed as Identified and the Value as Identifier, the clause is a decoding one …
if the Value is construed as Identified and the Token as Identifier, the clause is an encoding one …
In other words, the identity either decodes the Token by reference to the Value
or it encodes the Value by reference to the Token.