Arrow Technology allows over-usage. When you get closer to using up all of your disk space you will start receiving regular notifications. For a lower capacity plan that might be at 80% and for a higher capacity plan it might be 90%. That is an opportunity to remove old files, data and emails that are no longer required and contact Arrow Technology for assistance if required.
If you continue increasing your disk space usage you will ultimately exceed your allowance. Notifications will be sent for this, but you will be allowed to continue over using your disk space. Arrow Technology will discuss with you how best to resolve the situation, such as a web hosting plan upgrade or suggestions on reducing the usage.