<add> (addition) contains letters, words, or phrases inserted in the source text by an author, scribe, or a previous annotator or corrector. [3.5.3 Additions, Deletions, and Omissions]
In a diplomatic edition attempting to represent an original source, the add element should not be used for additions to the current TEI electronic edition made by editors or encoders. In these cases, either the corr or supplied element are recommended.
In a TEI edition of a historical text with previous editorial emendations in which such additions or reconstructions are considered part of the source text, the use of add may be appropriate, dependent on the editorial philosophy of the project.
Example
The story I am going to relate is true as to its main facts, and as to the consequences <add place="above">of these facts</add> from which this tale takes its title.