Deep Battle or Air Land Doctrine / whaatever is the idea to deny the enemy options by ATTACKING the reserves. It was the primary Soviet doctrine of WWII and the Chinese doctrine of the Chinese Civil War. Never mind the vanguard and/or the mainforce. once you hit the reserve3s, you deny any and all options the enemy can perform.

Thereforee, the envelopment of a meeting engagement IS NOT meant to SURROUND the mainforce BUT to kill the RES. That is WHY the Taiwan invasion is so tough. You HAVE to GET THROUGH an ENTIRE CORPS before you get to the RES. And Corps level RES is Bde, so you need a DIV to match it.