The Diamond reseller plan is a perfect place to upgrade when you are selling your personalized hosting solutions and have reached the full potential of the Gold plan.
The Diamond reseller plan is also perfect for individual users who do not want to resell, but do need multiple cPanels, IP's or SSL's. One reseller plan can be much cheaper than several shared webhosting plans.
When you have allocated all of your disk space and bandwidth, the next step is to purchase a second reseller plan to accommodate your needs, or upgrade to a dedicated server.