SQL_ASCII and UNICODE server_encoding

Hi, 
i'd like to ask if there is any difference in server_encoding 
SQL_ASCII and UNICODE (there must be
one since the different terms), but
AFAIU
-databases which store in SQL_ASCII (i.e. server_encoding=SQL_ASCII)have no problem storing 8bit chars with the first
bitset.
 
-databases with server_encoding=UNICODE actually store in UTF-8 encoding
-The purpose of UTF-8 is to make possible that multibyte chars can
be stored in systems that represent chars in 8 bits
-So SQL_ASCII is fine for UTF-8, hence multibyte UNICODE.

So if SQL_ASCII should be fine for UTF-8 storage.
(here i must have missed something, tho..), whats
the purpose of server_encoding=UNICODE?
-- 
-Achilleus