According to the manual:
Reserve: A group can have reserve time, which is extra time allowed in case the tasks inside end up taking longer than they are scheduled to take.
However, I can’t find “Reserve” in the properties for task groups, or anywhere else. Has this been removed? What do people use to add duration buffers to project tasks? I could add a task with no effort and a certain duration, but that seems like an awkward workaround.