A Blue Jackets Manual should help you out. It covers the differences of True bearing, and Relative bearing. Anything directly starboard(right) of your sub will be relative bearing of 90 degrees relative bearing. Directly port(left) will be 270 degrees relative bearing.
So if you are sailing a course due west true bearing of 270 and use your scope to see behind you the scope will read 180 relative bearing when viewing directly aft, even though on your compass directly astern will read 90 degrees true bearing.
I'm just glad it's 1940s so we also don't have to deal with magnetic north and true north
