The tab is basically an outline on Edge Cuts and some drill hits. Unfortunately use of edge cuts layer in footprints is not officially supported, so often lines are marked on silk screen or other layer and an arc must be drawn over them on edge cuts after the footprint is placed on board.
I don’t recall where I got this lib*, but here is an example :
However, for the purposes of the 3d display (original post), then any sliver connecting the boards will render them correctly. I thought I had an example but I can’t find it.
- ETA: I think this is the lib https://github.com/madworm/Panelization.pretty