you can read more info on the ODB++ ouput in the help file Manufacturing Inputs and output guide.
The option you select depend on who reads the output.
For PCB production you can skip part numbers and the whole assy section, as they are more for assembly.
For Assembly you need to add the components and select the appropriate manufacturing layers in the window.
Make sure you send a test board to your PCB producer and to your Manufacturer and test that all relevant information comes across.
Please check out if they are able to read v8.0 or only the older v7.0 of ODB++.
Metric is better than imperial, if you board has been designed in metric units. (follow the design units)
Always compress your ODB++ file and send it as it is generated.
Hope this helps
Just push the Help Button in the lower right corner.