How does one define on which tile an entity can be placed?

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How does one define on which tile an entity can be placed?

I was looking at the elevated rail support, but I couldn't figure out how it works.

So I want to make a roboport made of tungsten, that can be placed on lava. I just have no idea how to make it possible.
Like it would be allowed to be placed on lava and light oil from fulgora, but not in water or deep oil.

I'm using a deep copy of of the roboport, and set the tech, recip and item, that is fine, it is just the tile configuration that I don't quite get it.