In an identifying relation, the identity either encodes the Value (the higher level of abstraction) by reference to the Token (the lower level of abstraction), or it decodes the Token by reference to the Value (Halliday & Matthiessen 2004: 230).
So, in the identifying relation: expression realises content,
the identity either encodes content by reference to expression,
or it decodes expression by reference to content.
And, in the identifying relation: form realises function,
the identity either encodes function by reference to form,
or it decodes form by reference to function.
And, in the identifying relation: structure realises system,
the identity either encodes system by reference to structure,
or it decodes structure by reference to system.