Classifying vs Identifying

Halliday & Matthiessen (2014: 286):

So for example

my brother                 is       tall
Carrier = member                Attribute = class

means ‘my brother belongs to the class of people who are tall’. This specifies one of his attributes; but it does not serve to identify him – there are other tall people besides. The only means of identifying something by assigning it to a class is to make that a class of one. But if the one-member class is at the same level of abstraction as its member, we have a tautology: my brother is my brother.