The term branch off or branch off date is used by release engineers and release managers of OOo. With this term they want to identify the date when the release code-line is split from the general developer code line (see the picture). Until the branch off features, enhancements and general integrations can be done on the development code line. But it is important to be finalize all implementations until this date. This means features, enhancements and general integrations must be completed. On the release code-line only stopper issues are excepted. This should stabilize the builds/milestones until the final release. General development has to be done on the development code-line (DEV300).
When is the branch off?
The date of the branch off will be announced on the wiki page for the corresponding release. E.g. the page for OOo 3.2 release.