Hi, I am not a developer of any kind. However, I saw a work issue on the Lilypond gitlab page (lilypond#6772) about improving the appearance of Kievan notation. One of the participants in the discussion, Dan Eble, mentioned that "It would be nice to hear how someone who works with this material would ideally divide it."
I am someone who works with the material, and I can give some insight into the question, though I confess I don't have any idea how to make a gitlab account to comment. Hence why I am sending an email, in the hopes that someone can get this info to the people who need it. The "figure 8" shaped Kievan barline they are discussing is actually serving two different functions. It either serves as a fine, or it serves as a section divider. To put it simply, in this style of Kievan notation, a "score," in Lilypond terminology, begins with what looks like a plain bar line at the very beginning of the score (i.e. at the beginning of the staff, before the first "measure"), and ends with a "fine" bar line. Sections within the score will use the same "figure 8" symbol as the fine, but will not have the initial plain bar line on the succeeding "measure." These section dividers mostly exist because this is a point at which the choir has to stop singing and wait on the priest or deacon to say or finish some prayer, or else they have to wait for some liturgical action to be completed. Basically, the only thing distinguishing the fine divider from the section divider is the presence or absence of a plain bar line at the beginning of the next system. This is not necessarily the case in all square notation. There are a few different styles, and while I have scans of a lot of square note books from different eras and places, I haven't studied all of them in depth. However, the scanned book they are talking about in that conversation is the Synodal Obikhod of 1909, and therefore represents the most "official" version of Kievan square notation. Let me know if I can be of any more help. I'm a self-trained church singer, and not even a particularly good one, but I do actually use this material, and so I know some things that wouldn't be obvious to those who don't, and can figure out most other things with a bit of investigation.
