Basically, what does he mean by the word “relations”? What does that word mean? I think it means, a thing which inheres as an accident in something but whom’s essence is more about “direction towards another” than any other accident which is usually just a modification of a subject by itself and not modification of a subject via another thing per se.
This is such a good philosophical question.
I know you asked about Aquinas. But the question itself has broader implications.
Think of the Trinity. One theological view is that the triune God is nothing but “relations”.
Or intentionality. That consciousness is always about “something” - i.e., consciousness is a transcendental “relation”.
Or Aquinas’ teaching that creatures have a “real relation” to God while God does not have a “real relation” to creatures.
Or Hegel’s teaching on “internal” relations, e.g., “north” can only be defined in terms of “south” and vice versa.
Or the view of Ferdinand de Saussure that language is a system of pure differences, i.e., relations. The meaning of one word is “generated” by its “difference” from all the other words in the language.
Or Theology of the Body. That the human body has a “spousal” meaning, i.e., is essentially “related” to the “other”.
“Relation” may be a more fundamental philosophical category than “substance”.
Because without the “relation” involved in certain philosophical “distinctions”, there would be no philosophical vocabulary at all. For example, without “accident”, no “essence”; or without “essence”, no “existence”. Every philosophical “word” is caught up in “internal” relations with every other philosophical “word” (something like de Saussure’s differential theory of language).
You may recognize in all of this Jacques Derrida’s “differance”.