This is a holdover meta tag from mathoverflow.net which only supports research level math questions, and it generally isn't welcome here or anywhere else on the network.
See
http://blog.stackoverflow.com/2010/08/the-death-of-meta-tags/
- If the tag can’t work as the only tag on a question, it’s probably a meta-tag. Every tag you use should be able to work, more or less, as the only tag on a question. Meta-tags, like [beginner], [subjective], and [best-practices], are useless by themselves — they tell you nothing at all about the content of the question.
- If the tag commonly means different things to different people, it’s probably a meta-tag. In a cruel, ironic twist, the meaning of the tag [subjective] itself … is actually subjective. Ditto for [best-practices] and [beginner]. Best practices to whom? Beginner by what criteria? These tags are impossible to define by anything remotely resembling an objective metric. In comparison, the the meaning of tags like [java], [c#], and [javascript] are crystal clear to all but the nuttiest of nutbags.
and
