Full Idea
Some properties may not be universals, if they can only be exemplified by one thing, such as 'being identical with Socrates'.
Gist of Idea
If a property such as self-identity can only be in one thing, it can't be a universal
Source
Chris Swoyer (Properties [2000])
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.2
A Reaction
I think it is absurd to think that self-identity is an intrinsic 'property', possessed by everything. That a=a is a convenience for logicians, meaning nothing in the world. And it is relational. The sharing of properties is indeed what needs explanation.