Why an instance is 'a token of a type'

The organising principle of the cline of instantiation is attribution (Halliday & Matthiessen 1999: 14-5, 145).

Attribution is concerned with class membership.
Attribution + elaboration includes type–subtype relations.

But, the most delicate subtype has just a single member.
The relation between a category and its single member is identification.
The category and the member uniquely identify each other.
The member is the Token that realises the category Value.
An instance is the Token that realises the most delicate category Value.

This is the distinction between instance and subtype.