Full Idea
To avoid the outcome (possible in 'Categories') that the subject might be nothing at all, Aristotle insists that a legitimate subject must be separate and a 'this' [tode ti]. Forms and composites satisfy the revised criterion in different ways.
Gist of Idea
A subject can't be nothing, so it must qualify as separate, and as having a distinct identity
Source
report of Aristotle (Metaphysics [c.324 BCE], Z.3) by Mary Louise Gill - Aristotle on Substance Ch.3
Book Reference
Gill,Mary Louise: 'Aristotle on Substance: Paradox of Unity' [Princeton 1989], p.83
A Reaction
I take it that we would say that a 'this' is an entity which possesses 'identity', and is perhaps countable. For Aristotle being a 'this' seems to require a possibility of definition. This is a powerful Aristotelian thought, needed in modern metaphysics.