Low line or underline?

Indicator Punctuation Marks > Fullwidth low line
https://www.w3.org/TR/clreq/#indication_punctuation_marks

U+FF3F FULLWIDTH LOW LINE [＿] is positioned underneath proper nouns such as a person's name, the name of a place, etc.

This doesn't seem right to me, not least because U+FF3F is not a combining character. I think what we are actually talking about is underline, isn't it? (in which case this subsection about fullwidth low line should be moved to a new location)

My understanding is that the u tag in HTML is designated as the way to indicate such lines below text, again making me think that this is actually a type of underlining.

The note that follows indicates that a 'wavy low line' is sometimes used. Does that look like a combining character (for which i couldn't find anything in Unicode on a quick search), or like a wavy line (in which case it's a type of underline that CSS could specify)? (It would help to have a picture of it in clreq.)

Start by reviewing the cited CLReq “Indicator Punctuation Marks > Fullwidth low line” section and the 11-comment discussion. Compare the terminology and behavior of U+FF3F, HTML’s u element, and CSS underlining, including the question of wavy low lines. Done means the specification’s terminology, section placement, and any needed example or image are agreed and updated.