What is singular and non-singular values in the context of STL iterators?

Question

The section §24.1/5 from the C++ Standard (2003) reads,

Just as a regular pointer to an array guarantees that there is a pointer value pointing pa

Answer 1

Iterators can also have singular values that are not associated with any container.

I suppose that's its definition.

How and why dereferenceable values are always nonsingular?

Because if they wouldn't, dereferencing them would be undefined behavior.