Excerpt For extension packages being developed alongside a base package, to support base API changes for the extension, the base is represented by a proxy package using a different namespace prefix so that the base can be updated incrementally during the development sprint. (TBD)
Some changes to a managed package are irrevocable, in particular global class interfaces consumed by extensions and integrations. By using a proxy base package under an alternate namespace, changes can be tested in a complete development environment. If necessary, the proxy package can be discarded, and another alternate namespace chosen.