Transparency
From polycount
Additive Transparency
The flames on this burning bed are using additive transparency to keep the colors "hot."
See additive color model.
Alpha Transparency
Alpha Bit Depths
Subtractive Transparency
The x-rays on this light-table use subtractive transparency to make things under them darker, the way real x-rays do. The subtractive method isn't used all that often, so if you need it you should ask your programmer(s) if they can add it as a specific feature of the engine.
