This is the feature where you can see more player fleets in a port when it is clogged up with fleets with danger. Scarcely used till now but with introduction of skirmish, this will probably change (Plunder pages should fill up at last).
The implemented version is at this post:
viewtopic.php?p=1921#p1921Discussion continued after this point. Skirmish ideas was first discussed in this topic.