The usage state for this entity.
This object refers to an entity's ability to service more
physical entities in a containment hierarchy. A value
of 'idle' means this entity is able to contain other
entities but that no other entity is currently
contained within this entity.
A value of 'active' means that at least one entity is
contained within this entity, but that it could handle
more. A value of 'busy' means that the entity is unable
to handle any additional entities being contained in it.
Some entities will exhibit only a subset of the
usage state values. Entities that are unable to ever
service any entities within a containment hierarchy will
always have a usage state of 'busy'. Some entities will
only ever be able to support one entity within its
containment hierarchy and will therefore only exhibit
values of 'idle' and 'busy'.