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can someone explain to me what is the purpose of this function and what does do when applied to object?

http://msdn.microsoft.com/en-us/library/windows/desktop/bb147173(v=vs.85).aspx

Mapping FVF Codes to a Direct3d9 Declaration

FVF stands for "Flexible Vertex Format", and defines "what kind of" and "how much" data each vertex has. I am no expert on the subject, however I do know that

FVF_XYZ = the vertices have position coordinates
FVF_NORMAL = the vertices have normals
FVF_PSIZE = this is used by shaders to define the sizes of point sprites
FVF_DIFFUSE = the vertices have a diffuse color
FVF_SPECULAR = the vertices have a specular level
FVF_TEX0 = the object has no UV coordinates
FVF_TEX1 = the object has 1 set of UV coordinates
FVF_TEX2 = the object has 2 sets of UV coordinates
and so on...

All normal models need a position at the very least, and would be applied as:

but if you wanted normals, and a set of UV coordinates as well then it would be: