@vpelletier says:

    I do not think this is a "slave" instance, but just a sub-instance.

    "slave instance" is a way to pass parameter to a foreign instance. For example,
    frontend is an instance, and a slave instance is the rule given by an unrelated
    partition to make that existing frontend instance direct http://foo.com to 1.2.3.4:80 .

    OTOH, "balancer" is a sub-instance of ERP5's SR base instance (which is typically the root instance also).

/noticed-by @vpelletier (on nexedi/slapos!43)