Whenever a CDN link is requested, the content is delivered to the end user from the CDN location that is the closest to them. When this occurs, data transfer is used to download the file. The amount of data transfer used (typically the size of the file) is counted towards your data transfer limit.
When your data transfer limit reaches 80%, 90% or 100%, an email notification will be automatically sent to the Primary Admin Contacts of your platform, as set under Admin > Settings. A Customer Success Manager will contact you to discuss upgrade options for your account.