Okay, since the name of lawyers is being blackened ( ), I figured I'd jump in with a lawyerly interpretation.
We're looking at the interpretation of the language:
Source Quality and Level
if I understand correctly.
I think there's little doubt that any reasonable person would interpret this to mean that there are two statistics for any summa: "Source Quality" and "Level."
For judicial interpretation, we look first to the document itself. And the term "Source Quality" is used throughout the chapter (and, in fact, the whole book) as a single term of art. There is no reason given in the chapter why in this one case the term should be considered a single term, "Source Quality and Level." This is particulary true since throughout the book, summae are described as having two statistics: "Source Quality" and "Level." To interpret "Source Quality and Level" as a single term means that each summa would only have a single statistic ("Source Quality and Level"), which would run counter to everything written about summae in the core rulebook.
This is further supported by the very next line that says that the Summa Gain Limit is the Summa Level. If "Source Quality and Level" is a single statistic, then summae do not have a Summa Level, and the indication of Summa Gain Limit is meaningless. We always try and interpret laws or rules in ways that do not make parts of them meaningless. If there are two interpretation, one of which is consistent with the rest of the law, and one of which would render part of the law meaningless, we go with the interpretation that maintains all provisions of the law.
Further, we then go to what callen noted, i.e., that the portion at the beginning of the chapter that refers to your Advancement Total indicates that your Advancement Total is equal to Source Quality + Bonus from Virtues - penalty from Flaws. It makes no mention of any term called "Source Quality and Level." This is further evidence that "Source Quality" should be considered a term separate from "Level."
I'll also address part of your question which doesn't make sense to me. You ask whether Summa Source is "Quality" or whether Summa Source is "Quality plus level." But "Source" is not a term of art with respect to Summae. Nowhere in the rules that I'm aware of do they ever refer to a summa as having a "Source." They always refer to each summa (or any other book) having a "Source Quality." So I'm uncertain as to what you mean when you refer to a book as having a "Source." Books in ARM5 don't have a Source, they have a Source Quality. [sup]1[/sup]
In the end, the only reasonable interpretation that is consistent with the rest of the rules is that the terms "Source Quality" and "Level" should be considered separate terms, and that when the rules refer to "Summa Statistics" as including Source Quality and Level then mean "Source Quality" and "Level" as separate statistics. Any other interpretation is unsupported by the rules, and actually runs counter to what is disclosed elsewhere in the rules.
[sup]1[/sup] - I think this paragraph highlights the heart of the confusion. (Sorry, had to get a footnote in.)