Various internal commands need numbers as arguments. Usually you simply
give a decimal number, but you also can use these prefixes:

  + Positive number
  - Negative number
  % Binary number
  $ Hexadecimal number
  & Hexadecimal APTR converted to BPTR
  ! Hexadecimal BPTR converted to APTR
  ^ Filename containing number

See  EVAL  for more.