Making a new branch based upon an old branch is a common activity in Bazaar. As time passes, the developer is likely to collect a number of branchs that are highly related to one another, and have many of the same revisions. This results in redundant storage.
Shared storage is the optional process of overlapping the RCS history of two or more branches in essentially the same place. With related branches stored together, redundant history can be removed, resulting in much more efficient storage.
Bazaar provides centralized storage via repositories.
(The original specification was here)