From the point of view of a service provider (Arriva in this case), all buses should be full to maximise revenue. From a user point of view, buses must never become full, so as to be able to pick up all who want to use the service. If a bus becomes full it shows the route capacity is too low, so more buses are needed.

One cannot (must not) stick to the timetable. Indeed, the timetable should be adjusted daily/hourly: Reduce the time between buses on overloaded routes and increase the time between buses on underused routes.

If a bus starts full there should be an empty one right behind it!

Change the frequency of buses during the day and run extra buses for part routes when required from particular venues. Simple – do it for a better service! Of course, real-life rules may also apply, such as a limited supply of buses, drivers etc.