The revival of the singing of Gregorian Chant in the early part of the twentieth century owed much to the "Solesmes method" developed by Dom André Moquereau, using new scores including additional marks including the ictus, episema, dot and incise, not found in the early manuscripts.
This led to controversy, which was eventally broken with a new understanding of rhythmic nuances taught by Dom Eugëne Cardine (a monk of Solesmes) in his classes in Rome, and by Dr Mary Berry in Cambridge. Dom Cardine used the term "semiology" to describe his interpretation of the signs (neumes) of the earliest manuscripts.
One of the results of this work was the publication of the Graduale Triplex, a reproduction of the Graduale Romanum with the neums from the Laon manuscript printed above the modern square notes in black, and the neums of the manuscript of the St. Gall family beneath in red. Correct interpretation of the neums is the singer's basis for developing adequate performance of the Gregorian melodies, and this tends to cast doubt on the value of the vertical episema written under a note means it has a mild emphasis, like an accent mark. In my own limited experience, these marks are more of a confusion than a help and if I am preparing scores and have the time, I delete them.
If you are seriously interested in Gregorian Chant or thinking teaching it, it is well worth obtaining a copy of the Graduale Triplex - the book can be purchased on the internet. The Graduale is for the Novus Ordo mass, with the revised calendar. For the Extraordinary Rite, scores may have to be selected, copied and rearranged.