Monomorphic vs Polymorphic type theory
As far as I understand it, the notion of polymorphic set denotes sets which are primitive, in other words which are first-order objects that can be manipulated, and such that set membership is a predicate, elements can be members of multiple sets. In monomorphic sets, sets are interpreted as predicates on types, and membership of a type is a jugement. In particular, a term can only be a member of a single set (up to equality of types).

While polymorphic sets are the more traditional view, there is an argument that the way mathematicians actually reason is mostly monomorphic e.g. $\mathbb{R}\in 2$ is not a proposition that should make sense. These notes by Martin Löf may clarify matters a bit.