This is a (lightly edited) composition of the comments left above by @Scortchi, @whuber, @Glen_b, and Roland (I am marking this answer as CW). I believe they fully answer this question.
There is no practical distinction between "focus on" and "about". Migration occurs when the question is likely to be considered on topic at the destination site. Closure occurs otherwise.
In practice we often close questions just because they're not good enough to migrate to SO. Many such lack a minimal, complete, verifiable example. Not every programming-related question is on topic at StackOverflow.
At the least be sure you're familiar with the requirements of their help/on-topic before trying to migrate there. If you don't feel qualified to judge if the question is of sufficient quality for the migration destination, then don't vote for migration.