Selecting New from the File menu has an option unavailable from the New button on the Standard toolbar. That option is for a Multiple Fixture file. In this case, Setup can include other UCSs as well as entirely different part files.
Select the system units you’ll use for the multiple part document.
Choose whatever units match the parts.
When you create the new Multiple Fixture document, you select and orient the parts in the Multiple Fixtures dialog box.
The multiple fixture part can be either tool or part dominant. Tool Dominant files use the currently loaded tool every where possible before moving on to the next tool. It is a form of tool optimization. Part Dominant files cut every part completely (within the limits of the setup) before moving on to the next part.
Click Add to include parts in the multiple fixture part file.