Halliday & Matthiessen (2014: 286):
In attribution, some entity is being said to have an attribute. This means that it is being assigned to a class, and the two elements that enter into this relation, the attribute and the entity that ‘carries’ it, thus differ in generality (the one includes the other) but are at the same level of abstraction [unlike Token and Value].
Note that this also, therefore, characterises the difference between instantiation (attribution/generality) and stratification (identity/abstraction). Language is more general than register; context is more abstract than language. This is the fundamental theoretical reason why register ≠ context.