The tag ft.factorization-theory was created in 2016 by a user who did almost the totality of the 23 questions with it. It is formatted as a top-level tag, while it isn't. Could it be changed to factorization-theory?
This tag has empty tag info, seems to be concerned with the multiplicative monoid in rings and factorization question therein. Although it's not very clear to me what it's exactly (and doubt about usefulness of a tag used by a single user on a subject that is broader), I think the existence of this tag is harmless. But shouldn't be disguised as top-level tag.
Added: the use of "-theory" seems to make it appear like an well-established field, while I guess it's not the case for "factorization" which deals with some natural factoring problems in ring or semigroup theory. (Of course I consider such a notion as UFD well-established, but it's one specific instance and in any case the tag is not used in questions about UFDs).
For comparison, a quick view of tags with " -theory" (see the list of tags with 'theory'). It includes the top-level tags gr.group-theory, nt.number-theory, rt.representation-theory, category-theory and a few more, including less used sp.spectral-theory, it.information-theory (the latter comes from the computer science arXiv). Among the main non-top-level tags, it appears in widely used graph-theory, measure-theory, set-theory; let me mention as sample a few less used but still over 100 questions such as matroid-theory, descriptive-set-theory, conformal-field-theory. These subjects often are documented by textbooks, including more specific ones such as arakelov-theory. In a few cases the '-theory' might oversell the subject, or denote a not clearly defined subject (for one instance of the latter is algebraic-theory: actually it should be algebraic-theories in such case since the tag is about these theories as mathematical objects, rather than as mathematical field— by the way it seems to be entirely captured by universal-algebra but this is too much of a digression). We should pay particular attention when such tags are created).
Added: I realized while adding the previous paragraph that ft.factorization-theory disappeared (was burninated?) and was replaced with factorization-theory.