Bit Planes

  You can choose the number of colors available in ReGIS and
  Sixel graphics by entering the number of planes supported
  by your system.  For example, on a system with 2-bit planes,
  DECterm displays four different colors simultaneously; on
  a system with 4-bit planes, DECterm can display 16 colors
  simultaneously.

  By default, DECterm uses 2-bit planes on 4-plane systems
  and 4-bit planes on all other systems.  This reduces the
  probability that DECterm must create a private color map.

  Applications written for the VT240 terminal require only

  2-bit planes, while applications written for the VT340 may
  require up to 4-bit planes.