The mapping tag on TeX.SX has absolutely no definition or guidance. The questions about it are a grab-bag asking about everything from graphing a conformal mapping to the Mapping= feature of fontspec to the higher-order map function from computer science to character-set conversion to transliteration between scripts.
Is it doing anybody any good? If it’s worth saving, what should it be defined to actually mean?
I suspect that there are many tags like that. I'm of two minds about these kinds of tags. Another way of thinking of them that is different from the 'grab bag' characterization, is that they are context-dependent tags. Another one is the examples tag. This tag is often added to linguistics questions since the major linguistics packages are designed for formatting linguistic example sentences (a technical term of art in the field completely analogous to 'equation' in math or 'formula' in chemistry). But obviously in other contexts the tag will mean something else.
So we can think of a context dependent tag as one whose meaning will change relative to the other tags it appears with. Of course the tagging system isn't really set up like this, since tag descriptions are supposed to be unique categories. But if the tag is meaningful in more than one domain, albeit with different meanings it may still be useful for people so long as its combined with other more specific tags. And tag descriptions could be made to include some of the ways in which the tag can be used, noting the conjunction with other relevant tags.
Of course if a tag applies to far too many contexts, then it's likely not very useful at all. The current mapping tag has only 20 questions, and so doesn't fall into this category. Certainly the cases that relate to XeLaTeX and perhaps fonts more generally benefit from the tag, but maybe some of the more random uses can have the tag removed from those questions.
font-mapping and remove it from questions about conformal mappings or tl_map?