Currently, there are only 2 choices (Meta and SO), following this flag menu:
-> A community-specific reason -> This question belongs on another site in the SE
These choices are among the most frequent, but there are more cases where the question fits exactly ServerFault, SuperUser, Math, Softwareengineering, and others, or the same question exists already there (without being relative to Unix/Linux).
It would be useful to add more choices. Or one more choice, like "some other site", with or without the option to specify it (For SO a similar option exists).