Before we open up a semiological can of worms, I will try to answer the question as you have posed it. That is, how to interpret the notation found in modern chant books. I have said this before, but I will say it again:

The rhythmic signs (ictus, dot, and episema) mean what the people who put them there say they mean.

The episema, as it is used in modern editions, is not found in the earliest chant manuscripts. If you want to try to perform chant in a more historically informed way, you will not find the information you need printed in the square note only editions of the Vatican and Solesmes. (That editors of early polyphony have so universally adopted the chant interpretation of the late 19th century in a small region of France for otherwise meticulously prepared scores of music from the 14th-18th centuries continues to confound me). The way to perform the Solesmes notation is the method developed by its early practitioners, and laid out in several easy to obtain method books.

As a disclaimer, I have to say that the Solesmes edition of the chants and the methodology behind their performance are a major contribution to the Church, and only slightly less so to the field of musicology. But there is no mystery surrounding the meaning of their notation.