The Requirements subclauses may describe names that are used to specify constraints on template arguments.160 These names are used in library Clauses to describe the types that may be supplied as arguments by a C++ program when instantiating template components from the library.
Certain types defined in Clause [input.output] are used to describe implementation-defined types. They are based on other types, but with added constraints.
Examples from [utility.requirements] include: EqualityComparable, LessThanComparable, CopyConstructible. Examples from [iterator.requirements] include: InputIterator, ForwardIterator.