So for the title question, the answer is, well, the question is invalid. Neither sdist nor wheel distinguish between source and data files; a file is included if it’s in the archive and (in wheel’s case) listed in
RECORD. But since OP mentioned
include_package_data, I’m guessing the actual question is something like Is there a standard way I can tell a build tool what files to include when it builds my sdist and wheel. The answer for which is no, the rule of file inclusion is entirely a contract between you and the build tool (setuptools, poetry, flit, etc.), and there is not a standard rule tools must follow.
The reason is two-fold. First of all, the standard is not technically necessary, since it is never expected a developer would freely switch between tools anyway; if a project uses flit, its maintainers all use flit, and all the flit configs only need to make sense for flit. However, the same can be said for all project configurations, yet we still have PEP 621. The reason this was not included in PEP 621 is because PEP 621, as it stands, is already very long, and the discussion around it too diverging, we need to draw a line somewhere to limit the scope so the discussion and specification is managable, and file inclusion/exclusion logic is one of the things that are 1. not tightly related to anything else, 2. tools currently do things sufficiently different that a discussion around it can derail other topics, and 3. few people strongly want it standardised. So we decided to leave it out of PEP 621.
This does not mean there cannot be a standard for this, however. You can write a PEP for it yourself, and as long as setuptools, flit, poetry, etc. people can gather around and agree on a common logic, it will be standardised. That’s how every packaging PEP works.