Aristotle has just finished describing why accidents cannot be used to describe what something is. He continues his discussion of how to argue by describing the two errors that occur in problems. He does this in Topics, Book 2.
We must also define the errors that occur in problems. They are of two kinds, caused either by false statement or by transgression of the established use of language. For those who make false statements, and say that something belongs to a thing which does not belong to it, commit error- and those who call objects by the names of other objects (e.g. calling a plane-tree a man) transgress the established terminology.
Next, Aristotle discusses what kinds of errors happen inside problems. There are two kinds of errors: false statements and misuse of language. A false statement says that something is true of something else when it is not. The misuse of language happens when we use the wrong word to describe something. We should use words in the same ways that everyone else uses them.
Aristotle is going to use these kinds of errors in order to set up the various rules of argument.
Next, Aristotle discusses how to detect these kinds of errors by how to recognize when something else is mistaken for an accident.