Keep-out Layer

Currently I'm using Altium Designer for all my projects and, as the subject suggests, I'm wondering in which layer should I manage my Keep-out Layer? From your web page I can see you request the GTL, GBL, GTS, GBS, GTO and GBO layers and that in one of these layers I should add a board outline... but in the case of a USB connector or other devices that have a metal housing, I use the keep-out layer so that no copper is left under these components...

Any advice? I'm just waiting for this answer so I can send my pcb for manufacturing.

Additionally... should the gerber files be exported in inches or mm? and a 4:4 format is adecuate?

Regards.