secundum rationem = [here] according to reason. Here, moreover : in the mind.
While an accidental determination (expressed by a term belonging to some Predicament) adds some content to that of which it is said, a transcendental determination does not add anything to that of which it is said. If we say, for example "a being is one" (where 'one' is a transcendental), '(is) one' does not add anything to something that is a being, because being already implies unity (Being and Unity [and also Multitude] are convertible).
It is just that we cannot express Being (in the sense of a being) fully, that is, express all its proper aspects by one term or concept. So we say : Being is One, Being can come by a multitude of beings, Being is Good, Being is Beautiful, Being is True. But all these 'determinations' are equivalent to each other and to Being. If we say Being is Good, then we at the same time mean Being is One, Being is Beautiful etc.