In P1885, a registered character set is one that is in (at the point when the paper was written) the IANA character set registry. P1885 also provides static functions to query about the encoding used in either the translation or the execution environment. In some cases (involving subsets or supersets), there are questions of when an implementation should return a registered character set as the result of such static functions.

The environment-implements-superset case presents itself in relation to csBig5. The system encodings for "big5" on Windows and AIX contain characters that are not part of the common base of Big5; however, both are also missing characters from Big5-2003:

Big5-2003 has U+7881 as F9 D6 and U+2460 as C6 A1.

Windows has U+7881 as F9 D6 but not U+2460 as C6 A1.

AIX does not have U+7881 as F9 D6 but does have U+2460 as C6 A1.

So, the environment-implements-superset case can, in practical terms, be generalized as being about divergent implementations of "charsets".

Of course, that generalization could also account for some environment-implements-subset cases; however, in addition to more mundane reasons, the environment-implements-subset case also arises from a technicality: It is questionable whether or not a POSIX environment that uses a UTF-8 encoding paired with a 2-byte (UCS-2) wchar_t can be said to have UTF-8 as the environment text encoding because the characters outside of the BMP cannot (based on wchar_t-representability) be considered members of the character set associated with the environment.

Are the design goals better met or not by allowing divergent implementations of "charsets" to be identified as being the same registered character set?

When an implementation indicates a specific environment encoding, do the design goals require that all associated characters or members of the associated code space be wchar_t-representable?

It may be useful to characterize the questions as whether the result of the static functions are meant to be more of a hint (with few guarantees) or more of a promise.