I'm wondering how to tag How do I display line numbers in emacs (not in the mode line)?
It was originally tagged customization, but that's a really bad tag name. It can easily be confused with Emacs's framework for defining and setting configuration settings (for which I propose the tag name customize, but even that is going to be heavily misused). And it's overly broad: pretty much every question can be considered to be customizing Emacs in some way.
Linum mode is documented in the “Minor Modes” section of the manual, in the “Modes” chapter. But modes or minor-modes wouldn't do, because “show line numbers” isn't a question about minor modes, the fact that the answer uses a minor mode is an implementation detail. Under Emacs 23, that section was in the “Customization” chapter, but that too is not a good tag name.
So what tag can we use to categorize this kind of visual interface customization when editing or viewing files?