When overlapped, the inside boat reigns

RULE 

Part 2 - When Boats Meet

Section C - At Marks and Obstructions

Rule 18.2 (a) When boats are overlapped the outside boat shall give in the inside boat mark-room, unless rule 18.2 (b) applies

Buoy room rules do not apply at a starting mark surrounded by navigable water or at its anchor line from the time boats are approaching them to start until they have passed them... 

PURPOSE

Protect the rights of the leading boat.  It is assumed here that the leading boat is the one closest to the mark.