VPS and Dedicated hosting
If the limit of an account or email is about to be exceeded, then the system can automatically send a notification.
The cPanel user can go to cPanel and update the contact email. The icon is at the top and cPanel will send email notifications to you at 70% and 85% capacities for disk space, bandwidth, and email quotas.
You can also have WHM send email notifications to you at 70% and 85% capacities for disk space, bandwidth, and email quotas. Just add a contact email to each account when you create it in WHM.
You can enable/disable warnings and adjust the percentages in WHM > Tweak Settings.
Reseller hosting
The cPanel user can go to cPanel and update the contact email; the icon is at the top and cPanel will send email notifications at 70% and 85% capacities for disk space, bandwidth, and email quotas.
You can also have WHM send email notifications to you at 70% and 85% capacities for disk space, bandwidth, and email quotas. Just add a contact email to each account when you create it in WHM.
Shared Webhosting
You get unlimited capacities for disk space and bandwidth, so there won't be any notifications. You also cannot get notifications regarding email quotas.