The -unix switch specifies an alternate UNIX domain socket to be used for control messages. The default is /tmp/uml/ctl, but Debian changes this to /var/run/uml-utilities/uml_switch.ctl.
-hub specifies hub rather than switch behavior. With this enabled, all frames will be forwarded to all ports, rather than the default behavior of forwarding frames to only one port when it is known that the destination MAC is associated with that port.
-tap is used to connect the switch to a previously configured TUN/TAP device on the host. This gives a uml_switch-based network access to the host network.