Individual periods can be locked int eh substitution planning. This means that these periods are not to be shifted due to certain reasons. Select the respective period in the timetable and click on the lock icon in the toolbar. An "*" indicates that it is locked.
If you still want to shift the period or want to use it for a preponement, a warning message pops up saying that this period is locked. You can then decide, whether the period should nevertheless be shifted or not.