An unknown unicast frame is one where the location of the destination address is unknown, so the unicast gets flooded out in an attempt to find the destination address.
When enabling storm control with something like storm-control broadcast enable storm-control include-multicast unknown-unicast What counts as an unknown unicast packet?  Using a 5548 as an example, but the same question applies across the range of PowerConnects that we have.
Storm control, what is unknown unicast?