You probably don't need to save it at all.
If the cavity mill operations are in the same workpiece, turn on 3D IPW.
If they are in different workpieces, set the blank geometry type to IPW and select the other workpiece.
What should I do if I want to use IPW create a BLANK GEOMETRY? LOCAL IPW in "The part containing the source for the IPW must reference the same ....: Alerts are called will appear. Local IPW I set up what I want to know.
Can you explain your question some more?
Are you saving an IPW as a facet body?
Or are you using IPW flow with the blank selection on the geometry group?
For IPW flow, there are some rules about the components - search the help for "IPW Source dialog box".