If you have purchased more than one plan, each of them will show up on the main screen. If more than one plan is available to use in your location, they will be used up in the order that you bought them. In other words, the first data plan you buy will be the active plan—it will appear above the queued plan(s) and consumed first.
Should data from your active plan run out, it will disappear from the screen and the app will switch you to the queued plan’s allowance automatically.
In the case that only one plan is available in your current location, it is this plan that will remain active regardless of when you bought it. The other will be greyed out until you connect in a location the plan supports.