On Land

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At Rill Architects we run ArchiCAD on Mac OS X. If you work at Rill, this is your stuff. If you don't, but you work in ArchiCAD, you may find something interesting. Anybody else, I don't know.
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This layer sort of "forks off" from the layer S Slab.

Use it for framed structural decks (TJIs, 2x12s, etc). Continue to use S Slab for concrete slabs.

Why? Because decks aren't slabs. Yes, they're both made with the slab tool, but layers represent project elements, not tools. As much as possible, layer names should represent real things.

Where does this leave the layers A Deck2 and A Deck3? Right where they were. Model the deck surface on A Deck3. Model the structure of a deck on S Deck. Place 2D elements representing the (architectural) deck on A Deck2.