In general, it would be good to decide on what is going to be used here - is it "AtomPub" (Title Page, line 2, etc) or "APP" (line 42, etc)? If it's APP, and the title is going to continue to be "AtomPub" (rather than Atom Publishing Protocol), then at least be consistent and define the first use of APP. I don't have a preference - what does the industry use most of the time?