This sort option is available only in Newswire.
This option shows you Newswire stories that have accumulated the highest count of unique publishers who used your story. The different between this metric and the number of matches is that a Newswire story might have multiple matches from the same publisher.
Typically, the count of unique publishers is smaller than the number of matches.
The unique publisher count on your story card depends on your filter settings.
Example of a Newswire story when Unique Publisher sorting is selected. You can see the count at the bottom right.
You can find this option at the Sort ordering drop-down menu.