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Example of a placeable set to -50 meaning the edge of any visible boundary. Usually players can't see this far unless the fog distance is increased.
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Stackable / Unstackable
In the toolset by default placeables can be stacked, such as placing a book on a table. In these situations the placed item will rest at the Z axis of the bounding box for the placeable it is placed. Custom content creators who wish to have "transparent" placeables where they will not stack placeables can put an "a" and the end of the mesh name in the MDL file. The toolset will recognize this and allow placements to bypass that placeable and not stack. This is useful for planes of water like a stream, custom grass meshes, etc.