Actually, Free machining allows masking and the mask file can be used to do a separate tabbed perimeter path. A Depth limit needs to be set so that the 3D won’t go through. The tabbed path Start level is the distance from the surface to the already cut roughing depth.
My primary purpose for using a mask was to avoid having to unnecessarily machine the flat. I don’t know if it’s a bug, but when a perimeter Margin is needed a margin is added to the masked area and the G-code has to be manually edited to get rid of it 
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Mask files are easy to create using the v12 3D Covert to 2D Drawing > File : Edit drawing. With a mm STL and (Setup : Basic : editor set to) Inkscape it’s Manual scale (mm) > Save as (default svg, no changes/edits required). The only twist is that the SVG needs to be opened (Workspace > Mask) as Pixel 96dpi (Setup : Basic : Length unit = Ask).
