I think you should have posted this question to the DirectX/OpenGL/etc. forum, but since I''m a nice guy I''ll try to answer your question anyway

D3DVERTEX: a vertex that should be transformed and lit by the hardware.

D3DLVERTEX: a vertex that has already been lit, but still needs to be transformed by the hardware.

D3DTLVERTEX: a vertex that has already been transformed and lit and should be displayed "as is" by the hardware.

So, basically, the extra L''s and T''s just tell DirectX what you have done alreay to the vertexes.