What each setting is for
The settling-in period gives you ten or thirty seconds to sit down and close your eyes before the first bell, so the session starts when you are ready rather than when you pressed a button. Interval bells are soft and optional; some people use them to notice a wandering mind, others find them a distraction and leave them off.
The closing bell is the same as the opening one. If an interval bell would land in the last few seconds, it is skipped, so the end is one clear sound.