Both. The 180 degree rule just makes sure motion blur looks as intended and is a mostly artistic choice that can vary depending on the scene. E.g. for action se

Both. The 180 degree rule just makes sure motion blur looks as intended and is a mostly artistic choice that can vary depending on the scene. E.g. for action sequences or particularly smooth motion in a dreamy scene, you can break this rule. Or, if there's moving water, you might want to choose a particular shutter in relation to the preset frame rate. The overall frame rate gives you the distinction between a typical movie vs a TV-style documentary. The overall frame rate stays fixed across a movie and should normally not be changed.