The purpose behind the feature branch is to more formally stage a large pull request, set of pull requests or proposed architectual change to allow for for better review participation ( of the pr and changes due to feedback ) and more formal collaboration and testing.
Each feature branch may have a set of Confluence pages for describing the feature branch effort and goals, along with any other necessary items. Since feature branches may evolve into being from other discussions and efforts, there may be material items that are linked into the pages.
Original dev@metron.apache.org thread on speculative branches