Isn't Nvidia N12P-NS2 a codename for some graphics card? What is the normal name for it?
It is used in the description of this Lenove T420s: 4173-28U.
I expect it to be the Nvidia NVS 4200M. Is it correct?
This means that there are two GPUs: one integrated with the CPU, and an Nvidia circuit that kicks in when the software decides that it is needed. The Nvidia circuit on the Lenovo T420s with Nvidia Optimus is specified as Nvidia NVS 4200M, as you say.
The T420s is available both with and without Nvidia Optimus, though (with a large gap in cost, at least in my region).