Based on Monday's call and several issue threads (notably /wpub #11 and /publ-wg 21) we seem to have less than consensus about the difference between an identifier and a locator, and if there is a distinction, about whether the nature of this difference matters or is useful to highlight in the context of Web Publications (WP). Personally I think we need working definitions of identifier and of locator in WP terminology section. Below are my draft working definitions. But since it's unclear if the WG feels it necessary to distinguish between locator and identifier, before moving forward with a Pull Request, I would appreciate feedback about whether we need these definitions (as well as on the proposed wording). As a way to elaborate on the practical meaning of these definitions and to highlight the distinctions, I include comments below the proposed definitions. These comments are not intended for the terminology section of the spec (and might still need discussion), but may be helpful in drafting other sections.
Identifier
An identifier persistently identifies a WP or WP resource. A WP or WP resource MAY be identified by more than one identifier, but an identifier MUST NOT identify more than one WP or WP resource. Different versions or editions of a WP { SHOULD? MAY? } have distinct identifiers. Some identifiers MAY also serve as locators. URIs [[rfc 3986]], IRIs[[rfc 3987]], URNs [[rfc 8141]], DOIs, ISBNs, PURLs are all examples of identifiers frequently used in publishing.
Locator
A locator is a URL that can be used to locate and retrieve a WP or WP resource, subject to authentication / authorization and similar access limits mediated by HTTP or other protocols. Locators MAY rely for retrieval functionality on HTTP redirection and/or intermediate pages, e.g., a "landing page." Unlike identifiers, the locators associated with a WP or WP resource MAY change. Locators that persistently and unambiguously identify a single WP or WP resource are also identifiers.
Comments about identifiers: Identifiers are not required, but generally { SHOULD? MAY} be assigned to a WP. An identifier once assigned may not be reused in the future to identify a different WP. A single identifier cannot identify both a part of a WP and the WP as a whole; it must identify only one or the other. Some WPs and WP resources MAY be available in multiple representations (e.g., multiple serializations); an identifier identifies independent of representation. Some identifiers are also locators, but not all. Though URIs and URLs now share a common syntax, a distinction still made in this context (and in some other contexts) is that URIs identify while URLs locate. Other kinds of identifiers (e.g., DOIs) can be mapped (usually through services) to locators. Other kinds of identifiers (e.g., an ISBN) identify only, i.e., are not immediately useful for locating an online representation of a WP or WP component.
Comments about locators: Locators are required. The serialization, formatting and even byte-level content of a representation retrieved when dereferencing a URL may be fixed or may depend on content negotiation, the client device used to resolve the URL, the client IP address and/or other HTTP headers accompanying the Request, e.g., an editor using a browser or other tool that provides a HTTP Authorization header MAY see a different view than an anonymous reader. In some serializations, the resource (e.g., manifest) requested using a locator MAY be embedded within and conflated with another resource, e.g., a manifest conflated within the HTML representation of a WP's primary resource.