Concerning Alpha Transperancy
Alpha transparency can be created with 2 methods in DirectX and GTA provides flags to indicate which of the way is used. Those methods are independet from each other and can be combined. I am talking about alpha testing and alpha blending. Setting the flags 0x04 and 0x08 will enable alpha blending and/or testing. I am not quite sure what the parameters of the alpha testing function look like. As soon as I know I will update the article (in case anyone else knows feel free to edit the article anyway).
So alpha blending is a method that basicly depends on previously rendered texture information. This can also include vertex colors! The pixel get's an z-coordinate and get's passed to the frame buffer. Alpha testing will decide if the pixel is transparent or not, no matter if there is something behind the pixel, or not. The pixel does not get passed to the frame buffer - so alpha testing only knows transparent/not transparent which represents the way alpha maps are stored inside the TXD archives.