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If I have a multi-bodied part I need to first create several part files then individually export each as an STL to save parts for printing. It would be nice if I could easily export without creating several extra parts that I don't otherwise need. It would also be good to do this from an assembly.
You do not have to create any extra parts to deal with this scenario for multibody parts.
Multibody - How to save out a single body to .stp,... - PTC Community
For assemblies there is a config option that can enable export filtering.
config option intf_out_blanked_entities no
With this set to 'no' only the items that are visible on the screen will be exported.