It is a sign that indicates a specific recitation in at least another valid qiraa'a. It can also refer to a possible mad مد (a prologation in the recitation). It is not part if the Quran nor a valid sign of the rasm al 'Othmany of the moshaf. It is an addition to help the reader to read/recite the Quran as it was revealed.
I hope to find time to improve this answer.