In the Vivado timing report, nets are flagged with "unplaced", "estimated", or "routed". Some nets are labeled with "unset".
What does unset mean?
Figure 1 - Timing Report example with 'unset'
The "unset" label means that no possible placement is found for the driver or loads of the net.
In the above example, the input port xtclk does not have IBUF inserted, so the net is labeled with "unset".