The tag feature of Math Stack Exchange seems to be a nice feature but I don't understand why some are allowed and some aren't. For the example of umbral calculus, I tried the "umbral" tag. I can understand that the Sheffer sequences or Appell sequences are removed because probably too specific. Is umbral calculus that rare? With my question, some else also posted a different question about it, so I think it is not that rare to be removed as a tag.
My question is:
What are the requirements to create a new tag (power you acquire after 1000 points)? Does the frequency of a question matter or is it the importance of the field in research? How can people with 1000 points have access to data to make new tags correctly?
A side question being:
Why was "umbral" calculus not allowed? For me it seems decently large to have its own tag.