PEP 751: provide explanation for why `path` and `url` are optional (#4109)

peps/pep-0751.rst

@@ -1083,6 +1083,19 @@ simply defaulting to the baseline in tools without considering the security
 ramifications of that hash algorithm.
 
 
+------------------------------------
+Require a URL or file path for files
+------------------------------------
+
+Originally references to files were required, e.g., ``packages.sdist.url`` or
+``packages.sdist.path``. But at least
+`one use-case <https://discuss.python.org/t/pep-751-now-with-graphs/69721/34>`__
+surfaced during discussions about this PEP where statically specifying the
+location of files would be problematic. And in earlier discussions the idea of
+the location being a hint wasn't preferred. Hence the PEP now makes the data
+optional, but considers the locations accurate if specified.
+
+
 -----------
 File naming
 -----------