A resource accounting unit (RAU) is a basic unit of web site resource utilization on our network. It is currently defined as a memory usage integral totaling use of one gigabyte of memory for one minute, or the equivalent amount of CPU if a system is CPU constrained.
To provide a simplified example, suppose a single site is running on a dedicated server with 4 gigs of RAM. That server is "worth" 4 RAUs per minute. But if the site is only using half that server's resources (whether RAM or CPU -- whichever is greater since the server is "full" no matter which one runs out first) over a minute, then it would accrue only 2 RAUs during that minute. If the site becomes idle the next minute and has no requests, it accrues no RAUs during that minute.
RAUs are currently used in conjunction with server types that feature resource billing. If you are not using such a server type, RAUs used by your site are ignored and will not be billed. They are still calculated and presented for your information and for our use in capacity planning.