Mass hosting is a configuration in which you enable several low-traffic virtual servers. For example, an ISP that hosts many low-traffic personal home pages would fall into this category.
The virtual servers are usually URL-host-based and are in one of multiple virtual server classes, depending on the level of service provided. For example, you could have one class that allows only static content, and another one that allows static content plus CGIs.
Notice that the virtual server installed when you installed the server, VS1, still exists in defaultclass.