@Noah_Katz wrote:
I don't know what boundaries we're talking about.
If you invoke the boundary fill operation and select a solid body, you will be presented with a UI that requires you to select a cell. A single solid body usually only has one cell. So that should be pretty self-explanatory even for a novice user. After selecting the cell you can click OK and that will result in a copy of that solid body.
You can then select the boundary fill icon in the timeline and edit it. That would allow you to select another solid body.
If you copy/paste a body you'll end up with an icon for the copy-paste feature in the timeline.

That feature cannot be edited!
That can be a problem if you unstitch the solid body for a subsequent surface modeling operation, before the copy-paste feature.
The copy-paste feature loses that body selection and turns yellow. As it cannot be edited, even if you re-stitch the surfaces into a solid body, you'll lose all the work after the copy-paste feature.
This has been reported a long, long time ago and many times since then.