aCPU stands for Average CPU. In the shared hosting majority of customers have their sites hit very infrequently, often less than once a minute. Once your average CPU usage for such a customer across 1 day (or an hour or even 10 minutes), the actual CPU usage will often below 1%
This is why when you run lveinfo, you often see aCPU to be 0 or 1. If you see some user using 5-6% of aCPU, this user is using a lot of resources.
You can probably have no more than 10-15 such users on a server (15 users *5%=75% of total CPU resources of the server). So, such users should be either upgraded to VPS or charged correspondingly to their usage.
