Only in Cubase Artist is the defined Main bus fixed as the monitor bus. In Cubase you can definitely output any track to any output or mix you want. The defined “main mix” simply defines what the “default” output bus will be i.e. if you simply say “add new track” then it will be automatically routed to the predefined main mix - but you can change this. Also, the main mix bus is the bus that will be used by media bay etc. for previewing. If a main mix is not defined, you will hear no output when previewing - this is why you must still define a main mix in the outputs section even with “not connected” when using the CR.