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

Supplementary Ideographic Plane

    (SIP) The third plane (plane 2) defined in
   Unicode/ISO 10646, designed to hold all the ideographs
   descended from Chinese writing (mainly found in Vietnamese,
   Korean, Japanese and Chinese) that aren't found in the Basic
   Multilingual Plane.  The BMP was supposed to hold all
   ideographs in modern use; unfortunately, many Chinese dialects
   (like Cantonese and Hong Kong Chinese) were overlooked; to
   write these, characters from the SIP are necessary.  This is
   one reason even non-academic software must support characters
   outside the BMP.

   Unicode home (http://unicode.org).

   (2002-06-19)