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

and for tristate table http://www.cs.sfu.ca/~ggbaker/reference/std_logic/src/std_logic_misc.vhd

Truth Table

UX: if dataIn == U then U else X
DataOut*: Strength map for DataOut according to tristate table Buf3sTable