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:
convert object fvf objID,0x002
but if you wanted normals, and a set of UV coordinates as well then it would be:
convert object fvf objID,0x002||0x010||0x100