Both vertices and triangles use 2-byte indices and thus are limited to 64K. Most 3D cards today support 32 bit indices, but some old cards don't, that's why we still keep this limitation. Submeshes are just render passes of a mesh, and can not be culled separately.
Can you tell me the most recent 3DCard that does not support 32bit indices? Just for the interest...and for the question(if to old) of keeping the compatibility o.O.
Greets
Rackscha
