When we say "that being is a human being" then we signify (interpret) in the way of substance, that is to say, we let correspond with that being (that thing) a sign, and the nature and way of this sign and the correspondence, is a signification substantialiter (a signification in the way of substance).
Apparently we do this because that (particular) being presents itself (in observation) as an ontologically independent being, or, as KANT thought, because we impose this status (of independency) onto the thing or being. This latter possibility seems to me improbable (Nicolai HARTMANN offers a good argument against it), resulting in the conclusion that the cause of the relevant way of signifying resides in extra-mental reality and not in the conventions of our thinking, and also not in the a priori structure of our thinking (and observing).
Indeed, a thing is observable not until it has a qualitative aspect ('aspect' must here be interpreted ontologically). This is then at the same time the cause that one or another given being (thing) can be described in a qualitative way, that is, when we say "that being is red" we signify that being in the way of quality (that is, in a more external way than in the way of substance), apparently because that being has quality.
In this way we could nevertheless follow St Thomas that the Predicaments follow from the modus essendi (the way of being).
We can then say :
Unde oportet quod ens contrahatur ad diversa genera secundum diversum modum significandi (instead of praedicandi) qui consequitur diversum modum essendi ( Translation : So it is clear, that being contracts itself to diverse genera according to the different ways of signifying (instead of predicating), following upon the diverse ways of being).