I have no idea what they mean by "optional". It does not involve an additional mask layer. There are two masks associated with Vt adjustment, TN and TP. A native Vt device would be defined as the absence of both implants. The NAT layer is a drawn layer only; TN and TP are derived from COMP, NPLUS, PPLUS, and NAT. Of course GF could always charge the customer for the use of the drawn NAT layer, but then they would be douchebags, and anyway I have always assumed that NAT and native Vt devices are available. The magic tech file supports them. I have not designed anything using them.
