The inventory page is used to manage hosts and apps in your environment, however, hosts could really be used to manage any type of object.
Hosts are organized into sheets. In order to be part of an assessment, a sheet will need to be assigned to an OU. Each OU can have multiple sheets, and a sheet can belong to more than one OU. Using sheets allows you to logically organize hosts and makes working with large numbers of hosts more manageable.
Apps must have exactly one owning OU.