Bandwidth. Data is transferred across the internet each time a website is accessed, bandwidth can be monitored in real-time via control panels.
Max usage: 10GB on a rackspace.com datacenter server + 40GB on a nac.net datacenter server = total of 50GB a month per account.
Most clients use only our Rackspace Plesk servers but specific domains or downloadable files can be setup on CPanel control panel servers.
We don't charge per GB of data transfer, if you reach the account bandwidth limit you'll be invited to purchase a 2nd account. The average web page is about 50KB including images, 50GB is equivalent to over 1 million page views.