A nested tree is a directory tree that has been divided into two or more Branches.
One of the first question that people with nested trees ask is 'how do I replicate this nested tree?'. The typical response for a question such as this is a suggestion to take a look at ConfigManager. But plans are for bzr to support nested trees natively.
See also: Blueprint spec (NestedTreeSupport for details) and NestedTreeProgress.