I'm putting power into an small barn (about 40 amps) -- 300 feet from the house main panel. I want to use #6 Al URD to save $$ instead of 6-3 Direct Burial. Does the #6 Al URD need to be put in a conduit? Can I run it panel to panel?

URD is rated to be direct buried. Conduit would be wise to protect the cable from rocky soil, or under truck traffic, or to help replace wire in the future.

Any location the cable is not buried, such as rising into panels, will need conduit to sleeve the cable where exposed.