Your Capitan License provides your website with a bandwidth allocation, which is much the same as a data limit on a mobile phone contract. It's the amount of data which is transferred in/out of your website each month.

If you've been notified that you exceeded capacity don't worry - your service continues as normal. If your website exceeds the allocated bandwidth for 3 consecutive months your subscription will automatically upgrade to incorporate the boosters required for the new level of data transfer.

Upgrade prices can be found on the License Builder

How to check your allocated capacity and usage:

Login to your Capitan installation and to to "Settings", you will find your license usage and capacity on the right hand side of the page.