In Basic mode each button creates an event. It's a super simple and fast way of tagging but you are restricted to marking at most values for two attributes and optionally a note as well. You can have 15 of these buttons.
In Start & Stop mode you have a button for each Tree, Text or True/False type of attribute in your formula. You create an event and set the start and stop times yourself and can combine any type of tagging with the attributes as you choose.
Express mode is similar to Start & Stop but you have decided beforehand how long each event should be so you don't need to mark start and stop of the events.
Explained in more detail in this video: