This pertains to Rule 18 (rounding or passing marks)

Here is the rule:

18.2.e OVERLAP RIGHTS
[color:"red"] If there is reasonable doubt that a boat obtained or broke an overlap in time, it shall be presumed that she did not.[/color] If the outside boat is unable to give room when an overlap begins, rules 18.2(a) and 18.2(b) do not apply.

I'm confused about the highlighted section. One boat may have doubts that she obtained an overlap while another might have doubts that she broke an overlap. It seems like this doesn't really spell out who should do the avoiding if there is doubt about the overlap.