well it is the space occupied by idle virtual machines.
And the definition/settings for Idle VMs can be found in section 4a of Configuration->Manage policies screen
checkout this old blog post -> Rightsizing VMs using vCenter CapacityIQ – Part 1 | VMware Cloud Management - VMware Blogs
it's about CapacityIQ product (something that precedes vc_ops), but the basic concept for "idle vm" is still the same
hope that helps 