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This is a hypothetical, based roughly on a protest heard a few weeks ago.
Yellow and Blue are sailing in different fleet races with different courses. They are approaching a leeward gate which is a mark on Y's course but not on B's. B is on starboard, Y on port. If both maintain course contact will occur near the right hand gate mark, which Y intends to round.
The boats are overlapped at three boat lengths from the mark. B hails starboard. Y, entering the zone, hails for mark room. Is Y entitled to mark room (or any kind of room) from B?
Catalan, in my opinion Rule 18 applies only if the mark is to be left on the same side by both the boats. In the present situation Blue need not leave the mark on the same side as Yellow. Therefore rule 18 will not apply and neither 18.2(b).
Note "they" in the rule. So, no mark room.
There is no Mark Room to Yellow. Yellow to keep clear of Blue on Starboard.