"abi" version or "compatibility tag" of repositories
The idea is to add an “abi” version or “compatibility tag” of repositories. So if there are changes that are no longer abi compatible we need to change the abi version. System will warn a user if he tries to upgrade from imcompatible repositories. This will prevent “bad things” happen protecting a setup against disaster.
(from redmine: issue id 369, created on 2010-06-23, closed on 2013-02-15)