This function adds normal data (nx, ny and nz) as part of the new vertex format being created.
vertex_format_add_normal();
N/A
vertex_format_begin();
vertex_format_add_texcoord();
vertex_format_add_normal();
my_format = vertex_format_end();
The above code creates a new vertex format with just texture and surface normal values and then stores the format in the variable my_format.