See also: Agent introspection
juju_unit_status introspection function was introduced in 2.9.
In 2.9 the machine and unit agents were combined into a single process running on juju deployed machines. This tools allows you to see the status of agents running inside of that single process. Example output:
agent: machine-6 units: lxd/0: running neutron-openvswitch/0: running nova-compute/0: running