add example use cases, clarify 'no base_url' case, use "repodata_vers…

…ion": 2

cep-base-url.md

@@ -10,9 +10,13 @@
 
 ## Abstract
 
-Conda packages need to be stored in the same directory as repodata.json.
+Conda packages need to be stored in the same directory as `repodata.json`.
 
-This can be inconvenient especially when generating alternative repodata.json for the same set of packages.
+This can be inconvenient especially when using alternative `repodata.json` for
+the same set of packages. For example, a user might be interested in installing
+packages based on an older snapshot of the index data that points to packages
+from the original index. Or a user might want to subset `repodata.json` based on
+policy.
 
 ## Specification
 
@@ -26,7 +30,7 @@ A minimal `repodata.json` looks like this.
  "repodata_version": 1}
 ```
 
-Add `base_url` to the `info` object.
+Add `base_url` to the `info` object. Increment `repodata_version`.
 
 ```
 {"info": {"subdir": "...", "base_url":"https://repo.anaconda.com/repo/main/linux-64/"},
@@ -40,6 +44,11 @@ Append `base_url` and `package-name.conda` when downloading a package.
 
 `base_url` can be an absolute or a relative URL.
 
+In the absence of `base_url`, packages are loaded relative to the
+`repodata.json`. Index data without the new `base_url` will continue to have
+`"repodata_version": 1`, but index data including `base_url` will have
+`"repodata_version": 2`.
+
 ## Copyright
 
 All CEPs are explicitly [CC0 1.0 Universal](https://creativecommons.org/publicdomain/zero/1.0/).