Now that the Web Publication spec is "on the side", we have 3 specifications in scope: Publication Manifest, Audiobooks and Lightweight Packaging Format.

Publication Manifest says nothing the concept of Primary Entry Page, which is logical.

LPF introduces the notion of Primary Entry Page and exposes a processing model for extracting the Manifest from the PEP if included. LPF will be a Note, and its use will not be mandatory for distributing Audiobooks, so we also have to write very clear information about the PEP in the Audiobooks spec.

Audiobooks introduces the notion of Primary Entry Page in a specific section but does not specify that it may embed the Manifest. There is only an illustration of that possibility in https://w3c.github.io/audiobooks/#example-16.

The wording of the spec can give the impression that the presence of a PEP is mandatory for audiobooks. The only thing that contradicts this impression is "The primary entry page should instead, if present, ...". The rule is much clearer in the ToC section ("a table of contents SHOULD be included;").