We may not be in position of providing an absolutely clean definition (so maybe some editorial hand waving would be necessary) but…

§4.2.3.4.2 The item Element says:

Each item element in the manifest identifies a Publication Resource by the IRI [RFC3987] provided in its href attribute. The IRI MAY be absolute or relative. In the case of relative IRIs, the IRI of the Package Document is used as the base when resolving to absolute IRIs. The resulting absolute IRI MUST be unique within the manifest scope.

The intention is clear but what is the "IRI of the Package Document"? After all, the package document is part of a ZIP file, it is not really on the Web, ie, it is not clear what its IRI is.

Can we say something more precise about this?