Speedify helps you avoid going over your carrier's mobile data caps by letting you set daily and monthly limits on your cellular connection.
When you first run Speedify, it will suggest setting a default daily and monthly limit on your Cellular connection.
You can change these values at any time, by clicking on the circle for your Cellular connection on the main screen. The outer circle represents how much is left on the limits. Here the Cellular connection has used about one third of it's daily limit:
If you have an unlimited data plan with your carrier, you can turn off the switches next to the Monthly and Daily limits to allow Speedify to use as much cellular data as it needs. Or adjust the limits to something more appropriate for your plan:
When you reach 75% of the data limit, Speedify will warn you. When you hit the full data limit on a connection, Speedify will stay connected, but will stop using that network connection. You will be prompted to either let Speedify keep using the network connection anyway, or to stop using it. Of course, if that's your only Internet connection, that means it will take your device offline, but that's better than using more data than you have on your contract.
Note that Speedify only tracks data usage from when you are actually connected to the Speedify VPN. If you don't always use Speedify, you should set Usage numbers that are lower than your actual monthly cap.