According to its desc,
<add> "contains letters, words, or phrases inserted in the text by an author, scribe, annotator, or corrector". A bit later in the current tagdoc there is a note contradicting this ("The
<add> element should not be used for additions made by editors or encoders. In these cases, either the <corr> or <supplied> element should be used.") This inconsistency reflects a long-standing debate both in the TEI community and beyond. The original TEI position, reflected in the
<desc>, was to avoid taking a position, but subsequent users have taken the view (reflected in the note) that markup should always mark only what's visible on the page. I think we need to revise the text to make clear that both options are tenable, but that projects may well wish to be explicit about which they prefer. Certainly we should not continue to contradict ourselves.