This information is part of the Modelica Standard Library maintained by the Modelica Association.

Description in VHDL is given by http://www.cs.sfu.ca/~ggbaker/reference/std_logic/src/std_logic_entities.vhd

Truth Table

  *  = do not care
  U  = L.'U'
  0  = L.'0' or L.'L'
  1  = L.'1' or L.'H'
  X  = L.'X' or L.'W' or L.'Z' or L.'-'
  NC = no change

Clock transition definitions:
  1-Trns: 0 -> 1
  0-Trns: ~ -> 0 or 1 -> * or X -> X|U or U -> X|U
  X-Trns: 0 -> X|U or X|U -> 1