There are a lot of fixtures and things out there that are great models in 3d, but would look rediculous in plan because of all the faceting. So you use symbolic lines to draw the symbol in plan, cool. The only problem is that if you put it over something (i.e. a faucet above a sink or a toilet above a tile floor) it is transparent. You can't draw a symbolic filled region in a family so you're stuck. You can do an extrusion in the shape of the outline, but then it shows up in 3d.
Solution: You can't draw a filled region in the family, but you can nest a detail component into it. So copy the linework, create a new detail component family, paste the linework in place, then make a solid white filled region. Then nest the detail component into the family and properly align, etc. The family will look correct in 3D and will mask objects beneath it correctly