Changing the (top/bottom/left/right) margin(s) for a whole document or at least for a whole page. This is the rightful domain of margins, often accompanied by the geometry and/or typearea (package) tags.
Switching on/off first-line indentation for all or specific paragraphs. This is covered by indentation. (I'm not aware of any package-specific tags, though the
indentfirstpackage would fit in.)
Changing the (left/right) "margins" in the middle of a page, e.g. for lists, quoting environments, and theorems. Questions concerning this matter feature either margins or indentation or both tags, with no discernible pattern.
I think we should come to an understanding to which tag questions of the third type belong -- margins, indentation or even a yet-to-be created new tag. I suggest that indentation is in fact the appropriate tag, and ask you to advance your opinion.
(Note: From the 30 highest-scoring margins questions, 4 belong to the "middle-of-the-page" type. Assuming about the same fraction for the whole tag, adoption of my proposal would mean the necessity to retag about 20 to 30 questions. It is possible that Stefan Kottwitz, owner of the sole margins tag badge, would lose this badge at least temporarily. Assuming that at least 100 margins question would remain, the tag badge would not be destroyed, but "orphaned" the same way as it happened for the memoir badge.)