the manifest feature is not documented anywhere, so I assume it was added by mistake as it's functionality is pretty much what's described in the strict feature.
Created attachment 25435 [details, diff] Patch
Manifest is what devs are supposed to be using. It causes the creation of manifests. Checking should be via strict.
but currently checking is only done when you have FEATURES=manifest
Checking is enable via strict currently (last I looked in pre17 at least).