Exposition only in the C++ standard?

What does exposition only exactly means in the C++ standard? Does it mean that the private/protected members marked exposition only are required to exist by standard, or are they just a "suggestion" of implementation, and are not required at all?

Examples include:

std::error_code::val_
std::wstring_convert::byte_err_string
std::array::elems
std::move_iterator::current
std::reverse_iterator::current
std::ostream_iterator::delim
// And a lot of others