predicate type by topic is a specialization of predicate category. Each instance of predicate type by topic is a collection of predicates each instance of which, when used in an arg0 slot to form a sentence, forms a sentence that is about some particular sort of thing. For example, purpose predicate is an instance of predicate type by topic because instances of purpose predicate are used to form sentences about purposes or goals. Similarly, ComparisonPredicates are predicates that are about how one thing or type of thing compares to another, and so Cyc comparison predicate is an instance of predicate type by topic.