A plan is a set of hosting resources. Sometimes a plan is also referred to as a service package. Creating and offering different service plans/packages is a common industry standard in the Cloud hosting world. If you want to initiate your hosting business then you should know how to offer various services and resources to your customers. 

Basically a plan is a set of different resources and services you are offering to your customers. You can offer various types of plans to customers depending on their requirements. Once a plan is sold and your customer requests for some additional resource or service then it can be perfectly handled by Add-Ons feature available in this section. For example you sell a plan with a limit of 1 GB disk space. If later on, the customer demands for 1 GB more disk space under the same plan then it can be sold via Add-On. 

Also there are different properties of various resources such as a Mailbox is a resource and its size is one of its properties. So, you can easily manage users' resources and its properties through HC10 under this section. You can set different properties for different customers as well. This section provides complete management of plans, composite resources, add-ons and sold add-ons.