Ok here is the second installment in the things I did not understand from the blender manual and the gamekit book.
In the explanation of the face buttons the manual and the game kit says about the Add button.
"Add:
Faces are rendered transparent. The color of the face is added to what has already been drawn. Black areas in the texture are transparent, white are fully brite. Use this option to achieve light beam effects,glows or halos around lights. For real transparency ust the next option “Alpha” "
Ok I understand most of this. But WHAT color. The material color of the mesh? The vertex paint color you are applying with the brush? What.?
so, when the game engine is running that is everything opaque behind the current object and any add or alpha mode faces that have already been drawn
however, the 3d view acts different [because opaque faces aren’t always drawn first, and every face writes to the zbuffer, so a tranparent add mode face can obscure opaque faces if drawn first and is closer to the camera]