Relaxing the requirement for a build element to have one or more items
Fixed a couple of typos
Making sure the requirements for the coordinate system to be mapped to the output device is limited the output coordinate system (as referenced in other parts of the specification)
Attempt to convey the message that a 3MF model would be ultimately be manufactured but can be in an intermediate state which is still not fully defined (e.g. there's no item referencing the object resource yet)