How does the labeling square work in Cap Lab?
I'm creating Motorcycles in one of my factories and I'm selling it in my auto shop, I've got the layout sort of like thus:

It goes from the motorbike purchasing unit to an inventory unit, to a labeling unit and finally to a sales unit, however it never actually goes from my inventory unit into the labeling unit so it never ever makes it into the sales unit either.
Obviously I must not know how to properly use it. Am I leaving a step out of the process? Am I simply not using the labeling unit correctly and if so, what am I doing wrong with it?
Thanks