Remarks
Subfunctors in general are like global versions of subsets. For example, if one imagines the objects of some category C to be analogous to the open sets of a topological space, then a functor from C to the category of sets gives a set-valued presheaf on C, that is, it associates sets to the objects of C in a way which is compatible with the arrows of C. A subfunctor then associates a subset to each set, again in a compatible way.
The most important examples of subfunctors are subfunctors of the Hom functor. Let c be an object of the category C, and consider the functor Hom(−, c). This functor takes an object c′ of C and gives back all of the morphisms c′→c. A subfunctor of Hom(−, c) gives back only some of the morphisms. Such a subfunctor is called a sieve, and it is usually used when defining Grothendieck topologies.
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