Requested feature overview description.
In the Tags section, allow any tag to be marked as Abstract.
Similarly to how tags have a Type field, a new boolean field Abstract should be added with means to edit this field. Editing this field would be restricted.

Why would it be useful?
On its own it's not very useful. But it's a requirement for some more advanced features.

What part(s) of the site page(s) are affected?
Tags section.