As you can see from the slide, in identifying clauses, the ergative function of the participants varies according to the direction of the coding.
The constant is the Identified whose ergative function is Medium (the entity through which the identifying process unfolds), whether the clause is encoding (when it is Token) or decoding (when it is Value).
However, the ergative function of the Identifier is Agent (the external cause of the identifying process) when the clause is encoding (and it is Token), but Range (the domain delimiting the boundary of the identifying process) when the clause is decoding (and it is Value).
[Halliday (1994: 166-7) & Halliday & Matthiessen (2004: 292)]