I'm sure that this is somewhere on MMO, but my searching hasn't turned it up.
When I flag a question as off-topic, two of the options that I am given are "This question does not appear to be about research level mathematics within the scope defined in the help center." and "MathOverflow is for mathematicians to ask each other questions about their research. See Math.StackExchange to ask general questions in mathematics.". These seem so similar that I cannot figure out when one might apply but not the other, so I always pick the first. (How) should I distinguish between when to use one and when to use the other?