Every event has its own level of importance, which may be from 0 to 5. A higher level means bigger and bolder font for day number in Calendar, so that you can clearly see your duties with a single look. If you don't like default colors for levels, you can change them in Config. Level does not affect alarms. It only affects visual presentation of events in Calendar and helps you to filter events in Report.

Tip: For less important events you may set their level to 0 and choose color for level 0 (do that in Config) to be the same as the font in Calendar. That way, these events will not be too showy in Calendar but you will be able to see them in Viewer and as Hints in Calendar.