Various bugs on Launchpad have "[FFE]" and/or "[UIFe]" in the title.
What do these acronyms stand for?
Features that are added to the development cycle is frozen at some time in order to ensure that the developers focus on fixing any new bugs introduced into the development release. This is Ubuntu's way of quality assurance since Ubuntu has a relatively rapid release cycle compared to Debian's.
User interface freezes are in place to allow translations and documentation team to effectively complete their part for the development release. If there are new features and strings being added to the development release, the work done by translators and documentation team will go obsolete and also ensure that the development release does not have a chance of being fully documented and translated. In order for that not to happen, User interface freezes are in place.
However, for any particular process, there always happens to be an exception and it is not different for Ubuntu's development release cycle. There are several Freeze exception processes in place to ensure that features deemed important still make their way through to the development release and this is achieved through Feature freeze exceptions and User interface freeze exceptions.