Type predicates take a single parameter and test whether it's a specific kind of object:

• [Number x] is true when x is a number
• [String x] is true when x is a string
• [Tuple x] is true when x is a tuple
• [FeatureStructure x] is true when x is a feature structure

Because the sets of numbers, string, tuples, and feature structures are effectively infinite, these predicates only work in input mode and so are deterministic.  So the x above can't be an unbound variable.  You can say [Number 7] or [Number foo], although there would be little reason to because the answer would be obvious.  But you can only say [Number ?x] when ?x has already been given a value.

There are a few type predicates, however, that can work in both input and output mode because there are relatively few objects of those types.  The predicate CompoundTask is true when its parameter is a task defined by methods (isn't primitive).  Since there's a relatively small set of these, it can operate in both input mode (test if an obejct is a compound task) and output mode (enumerate the existing compound tasks).