Why std::errc is identified as error_condition while std::io_errc and std::future_errc are identified as error_code

I just noticed std::errc specializes is_error_condition_enum<> to make it identified as error conditions, while std::io_errc and std::future_errc instead specialize is_error_code_enum<> to be identified as error codes.

Is this discrepancy by design or ?

Moreoever, what exactly does portable error code of error_condition mean ? because enumeration values of std::errc are definitely varied on Linux and Windows.