Packthrus are used bels that do not have a netlist cell associated with them. They are typically added to legalize routing to reach elements that would be otherwise unroutable or to make routing easier. Packthru LUTs will use one output and up to one input. Other parts of the LUT are still usable if required. A register will only be used as a packthru when the site is not required for other uses.
In the following example, a packthru LUT is configured to drive a logic 0 into the LOOKAHEAD bel from the O6 output of the LUT. The O5 output is free and a smaller LUT can be added to use this BEL.