I've seen thou used in mechanical engineering circles, but almost exclusively mil in electrical engineering. I've not found it too ambiguous in actual use, but I get what you're saying.
How would I go about making a PCB where one of the prepregs has a cutout?
golden_labels: appreciate the time you've taken to explain. Learned something today
VK3DRB: I think you'll find that anyone who does PCB design will know exactly what 'mills' are. I understand your dislike, but that's just the way it is.
Of course it makes sense to point to specific regions of the PCB using embedded text in a Gerber layer. So, what justification other than locking the embedded text to the design, is there to use general embedded text in a Gerber layer, over using a README file?
The problem is that the people processing the gerbers will need to be able to understand what is in the readme file. And the preparation process needs to include reading the readme file during processing.
What works best is to adhere to what the board manufacturer prescribes. ODB++ is a nice container format for production data. In there you can also specify the layer built-up for example.