Aristotle has just claimed that infimic species and primary substances are equally substances within those categories. He continues the Categories by showing that only species and genera of primary substances are secondary substances.
It is reasonable that, after the primary substances, their species and genera should be the only other things called secondary substances. For only they, of things predicated, reveal the primary substance. For if one is to say of the individual man what he is, it will be in place to give the species or the genus (though more informative to give man than animal); but to give any of the other things would be out of place—for example, to say white or runs or anything like that. So it is reasonable that these should be the only other things called substances. Further, it is because the primary substances are subjects for everything else that they are called substances most strictly. But as the primary substances stand to everything else, so the species and genera of the primary substances stand to all the rest: all the rest are predicated of these. For if you will call the individual man grammatical, then you will call both a man and an animal grammatical; and similarly in other cases.
Only the species and genera of primary substances should be called secondary substances. One reason for this is that only the species and genera of a thing reveal the primary substance. If we describe what a particular thing is, we do so by giving its species or genus. Saying anything else would not describe what it is. Imagine we were describing a plant: saying that it is green does not say what it is. Similarly, saying that is growing, has flowers or anything else does not describe what it is either. (I am assuming that we are giving the answer rather than playing some kind of guessing game.) The second reason is that the species and genera can be the subjects of everything predicated of primary substances. Primary substances are called primary because every subject is predicated of some primary substance or other. The genus and species are similar to primary substances because subjects predicated of the primary substance can also be predicated of the genus or the species. Some examples are helpful. We can say that Plato is wise. But that means that a man is wise and that an animal is wise. This is something that we cannot do with anything else. Plato is white. Plato is also short. But that does not mean that whiteness is short. That sort of claim does not even make any kind of sense. If Plato was sitting, then the claim that whiteness is sitting or that sitting is white cannot be made either. Only species and genera can do that.
Aristotle does not claim that differentia are secondary substances. Although he speaks of them later, he implies that they are not substances in this passage.
Next, Aristotle shows that no substance is ever in a subject.