While Anytime Data is used at any time of the day, when you have Daytime Data available only that will be used between 8AM and 6PM. When you run out of Daytime Data, your Anytime Data will then be used during that time.
8AM to 6PM - Daytime Data, then Anytime Data when your Daytime Data has been exhausted, then FAP when both data banks have been exhausted.
6PM to 8AM - Anytime Data, then FAP when your Anytime Data has been exhausted.
Hope this helps.
Edit: The Business Plans are structured this way because most of the data is used during the daytime with Business Plans.
