sql92 character sets

For my own amusement I'm reading the sql 92 spec about character sets. 
There are some concepts that are a bit difficult that maybe someone can 
explain for me:
  character set  character repertoire

for example in 4.2.1 it says:
 A character set is described by a character set descriptor. A  character set descriptor includes:
                                                           -  the name of the character set or character repertoire,
                                                                             -  if the character set is a character
repertoire,then the name       of the form-of-use,
           -  an indication of what characters are in the character set, and
                                     -  the name of the default collation of the character set.
 


What I have understod so far is that form-of-use is the encoding. So if 
the character set is UNICODE then the form-of-use could be UTF-8, UTF-16 
and so on.

The character repertoire however I don't have an intuition about it all.



Then we have this little section:
 The <implementation-defined character repertoire name> SQL_TEXT specifies the name of a character repertoire and
impliedform-of- use that can represent every character that is in <SQL language character> and all other characters
thatare in character sets supported by the implementation.
 

Had unicode been a superset of all character sets, then one could just 
have used unicode for SQL_TEXT. Exactly how do we create a character 
repertoire that can store any character from any character set.. Storing 
the character set for each character is not such a cool thing to do 
even if it would work :-)

SQL_ASCII in pg is similar, it's basically a number of bytes. But the spec 
seems to say that one should be able to count the characters as well (not 
the bytes) so SQL_ASCII is not the same as SQL_TEXT.

ps. This is not me volunteering to implement all this :-)

-- 
/Dennis Björklund