Sizes, Perpective Patch
It is possible to define size of a mesh like one object, or you can define the size of the patches that are in a mesh.
Sometimes, our mesh has more than one patch, and each patch has a different perspective, so the size are different.
3dgrid alow to define different sizes inside a same mesh. Why ? Because we can to make difficult objects in only one Mesh !!
Perpective Effect is another function that we have in our meshes to simulate textures in a realistic enviroment.