The title resumes my question well: what does a white/hollow punctum represent in Gregorian notation?
From: les Bénédictins de Solesme. (1959). Paroissien romain, contenant la Messe et l’Office: N° 800. Desclée & cie.
From: Salve Regina (with Drone). (n.d.). Mass Propers. Retrieved August 7, 2022, from https://bbloomf.github.io/jgabc/propers.html#custom1=litany/salve-regina-drone