Join surface means you can specify the visibility of the common edges of two surfaces with the same texture property.
How to use it?
Join surface calculation for the entire 3D model can be a time consuming process.
Therefore ARCHline does this calculation for the entire 3D model in a 3D view when it is very necessary:
-When you change something that concerns the 3D model, the join surfaces calculation is not executed (the update of the 3D model is faster).
-However, when you use the Redraw (Ctrl+R command) in the 3D view or you regenerate the entire 3D model, the join surfaces calculation is executed.
You can accelerate the join surface calculation if you set up the filter for the most frequently used entity types. (File menu / Options / Display panel)