The prose isn't PEP-compliant yet, but here is my whole proposal for adding extras to .dist-info-style / Metadata 1.2 distributions.
An 'extra' is an optional feature that can augment a distribution's requirements. An extra name must be a valid Python identifier and is referenced using subscript notation, e.g. distribution[extra]. As a shortcut, multiple extras can be referenced as a comma-separated list distribution[extra1,extra2,...] which is expanded to "distribution distribution[extra1] distribution[extra2]". (It is not legal to require an extra without also requiring the providing distribution).
The name 'extra' is added to the list of variables in PKG-INFO environment markers. During marker execution 'extra' is either 'None' or the name of the extra that is being installed. (The markers must be evaluated once for each extra that is being installed.)
Extras are declared with the new Provides-Extra: extra field, repeated for each extra that a distribution provides. A distribution need not reference Provides-Extra: in any Requires-Dist: fields, a case that will most often occur to preserve compatibility with other distributions that require the named extra when a once-optional dependency becomes standard.
The extra names 'test' and 'doc' are reserved for requirements that are needed for running automated tests or building documentation, respectively.
Example lines from PKG-INFO:
Requires-Dist: nose; extra == 'test' Provides-Extra: test