Full Idea
The primacy of 'Categories' primary substances is a kind of ontological primacy, whereas the primacy of form is a kind of structural or explanatory primacy.
Gist of Idea
Primary substances are ontological in 'Categories', and explanatory in 'Metaphysics'
Source
report of Aristotle (Categories [c.331 BCE]) by Michael V. Wedin - Aristotle's Theory of Substance X.9
Book Reference
Wedin,Michael V.: 'Aristotle's Theory of Substance' [OUP 2000], p.453
A Reaction
'Structural' and 'explanatory' sound very different, since the former sounds ontological and the latter epistemological (and more subjective).