Related logic is defined as being logic that shares connectivity - e.g. two
LUTs are "related" if they share common inputs. When assembling slices,
Map gives priority to combine logic that is related. Doing so results in
the best timing performance.

Unrelated logic shares no connectivity. Map will only begin packing
unrelated logic into a slice once 99% of the slices are occupied through
related logic packing.

Note that once logic distribution reaches the 99% level through related
logic packing, this does not mean the device is completely utilized.
Unrelated logic packing will then begin, continuing until all usable LUTs
and FFs are occupied. Depending on your timing budget, increased levels of
unrelated logic packing may adversely affect the overall timing performance
of your design.