There are three types of override: Teachers, Normal Classes and Combination Classes.
For each, you can specify that they must be placed in a certain period, or must not
be placed in a certain period. Instead of a single period, you can specify a range
of periods in a range of days.
An example could be that heads of department should not teach on a specific period when
they are required for a scheduled meeting. An override would be inserted for each of
of these teachers indicating that they are not to be scheduled in that period.
Another example could be a teacher that cannot teach in the last two periods of the day.
A single override can prevent that teacher from being scheduled in those periods.
Once again, overrides should be used sparingly, particularly when constructing your first
timetable. Many constraints on the timetable can impair the overall quality.