System Generator: a block similar to a three state logic
If you mean 0/1/Z, which I assume from the tags, that's in the IOB.  Internal tri-states aren't in current FPGA families.  See the Xilinx documentation or any VHDL/Verilog book.

Specifically to answer trayres comment, there are external (IOB) tri-states, but not internal tri-states.