Fallacies of Ambiguity involve some confusion over meaning. Interpreted in one way the argument has a false major premiss, while interpreted in the other way the argument has a false minor premiss. (If we try to interpret both premisses so that they are true, then the "argument" fails to be formally valid.) Here is an obvious example:All beetles have six legs.
John Lennon is a Beatle.
Hence, John Lennon has six legs.
Since the location of the fallacy is a matter of interpretation, we cannot classify the argument either as a major-premiss fallacy or as a minor-premiss fallacy. Hence such fallacies get their own group.
A categorical syllogism is, by definition, an argument with three categorical terms. "Term" is to be understood in a semantic sense, as opposed to the syntactic sense of "word" or "phrase". In other words, it is the meaning of the words that is important. So, two different words with the same meaning are the same term, and the same word occurring twice with different meanings is two distinct terms. An argument commits the Four Term Fallacy which appears to have the form of a validating categorical syllogism, but has four terms.