In stroke play, an entire field must play by the same Rules and every player is responsible for protecting the rights of the field. So when something happens where you are not sure how to proceed, rather than having to make a decision that might be wrong or delay play to get a ruling from the Committee, there is a Rule that allows you to play two balls into the hole and then report the situation to the Committee for the final ruling.
To use this Rule you:
You don’t necessarily get penalized if you forget to tell someone which ball you want to count, but the Committee will have to default to the first ball you played and you may not get the relief you were asking for.
The most important part of this Rule is that you must inform the Committee about the situation prior to returning your scorecard. Failure to do so will get you disqualified, even if you think you scored the same on both balls.