Aristotle has just claimed that all affirmations are either true or false. At this point in the Categories he returns to reference and gives extra detail on the category of substance.
A substance—that which is called a substance most strictly, primarily, and most of all—is that which is neither said of a subject nor in a subject, e.g. the individual man or the individual horse. The species in which the things primarily called substances are, are called secondary substances, as also are the genera of these species. For example, the individual man belongs in a species, man, and animal is a genus of the species; so these—both man and animal—are called secondary substances.
The word ‘substance’ is used primarily to refer things such as individual horses and men. These things are not a part of something nor are they in something else. These are primary substances. Primary substances are certain kinds of things. These species and the genera of these species are all secondary substances. For example, Aristotle is a human being – the kind of thing that Aristotle is. But human beings are a kind of animal. So human beings and animals are both secondary substances of Aristotle.
This passage very clearly describes what primary substance and secondary substance refer to. Aristotle has not described how many secondary substances a particular primary substance might have, nor does he give a means of determining that. His only point in this passage is to distinguish between two different uses of substance.
Next, Aristotle claims that necessary predication applies only to names and definitions.