U is for unarticulated
The "u" element is for identifying a word or phrase as being flagged for the readers attention, or marked as significant, or silently annotated.
(The "u" stands for unarticulated, that is, not spoken or expressed in words.)
By convention the "u" element is styled with a solid line underneath the phrase, but other stylistic treatments, such as a wavy line may be better suited for this.
- The "mark" element is for highlighting a notable word or phrase within a sentence. This is typically accomplished by using a different colored background.
- The ins element is used during editing and proofreading to signal the addition of new text added to the author's original composition.