Importing and Exporting Vehicle Units
Similar to how Vehicle Components can be reused, Vehicle Units can also be reused.
When building longer combinations you may have identical trailers! You do not need to explicitly export the Vehicle Unit to reuse. If you only want to reuse the trailer in this combination, you click on the Swap Vehicle Unit button and then you will be able to choose a Trailer from the Vehicle.
Note: You can only copy down the combination, so you would configure your first trailer and copy it down.

Keeping a Vehicle Unit locked has benefits. You can update the Vehicle Unit to follow changes of it's original.
So on a longer combination you can just edit the first trailer and update the following trailers to the recent changes.
When a Vehicle Unit is locked, it's payload can be adjusted but no other properties.

Exporting a Vehicle Unit will create a Dependent File and may block you from making changes to that Vehicle Unit as that Vehicle Unit could be used elsewhere.
If a Vehicle has any dependent files, then swapping Vehicle Units will be disabled as that changes the Vehicle and Changelog. You will need to Save As and then swap the Vehicle Unit after.
You can swap Vehicle Units with the same button that Vehicle Components are swapped with. And next to that button you can export your Vehicle Unit explicitly.
Vehicle Units are stored individually under the Components Tab but you can also import Vehicle Units directly from Vehicle files if it is a one-off situation.
You can also create Vehicle Units by just adding in the Trailer or Truck as the first Vehicle Unit and saving then. When on a Rigid Truck or Prime Mover you will be asked if you want to save it as Vehicle Unit or Vehicle as Rigid Trucks can be a 'Vehicle' on their own.