(1) No risk of collision -- do whatever you like
(2) Risk of collision identified -- standing on is compulsory
(3) Give way vessel identified as not taking action -- stand on vessel has option of taking action
(4) Give way vessel cannot avoid collision -- stand on vessel must take action.
The problem is identifying when you cross the line between the different phases.
The first one is relatively easy. We have legal precedent that says six miles is in phase 1
But at some distance less than 6 miles, we move into phase 2.