The Free On-line Dictionary of Computing (30 December 2018):

eight-bit clean
8-bit clean

    A term which describes a system that deals
   correctly with extended character sets which (unlike ASCII)
   use all eight bits of a byte.  Many programs and
   communications systems assume that all characters have codes
   in the range 0 to 127.  This leaves the top bit of each byte
   free for use as a parity bit or some kind of flag bit.
   These assumptions break down when the program is used in some
   non-english-speaking countries with larger alphabets.

   If a binary file is transmitted via a communications link
   which is not eight-bit clean, it will be corrupted.  To combat
   this you can encode it with uuencode which uses only ASCII
   characters.  There are some links however which are not even
   "seven-bit clean" and cause problems even for uuencoded data.

   (1995-01-05)