my understanding is the MPU determines the scale of the scene. different OV apps may have the default set differently, but can be changed on the fly. for example, if MPU of your root layer is set to 1.0, the scene units becomes meters; conversely, if it’s at 0.01, it’s in centermeters. the unit of your scene can also be seen under the gimbal gizmo in the lower left hand corner of your viewport:
that said, if you were to bring a USD stage that’s in centermeters into another stage that’s in meters, there’s a conversion that takes place automatically thanks to another extension - the metric assembler ( Metrics Assembler — Omniverse Extensions latest documentation (nvidia.com))
if you find yourself using one specific unit as opposed to another and don’t want to keep switching it through the root layer property, you can set it in the preference of the app moving forward. i believe this preference will not alter the scene scale of the stage you may have currently open; it only affects the scene scale if you were to create new scene.
i am sure the mod will help clarify that further for you.