Datasheets specify the Rds(on) of a MOSFET at some specific gate-source voltage, e.g. 100mΩ @ Vgs=4V. Up until now, I assumed that the test gate-source voltage was essentially arbitrary and chosen depending on the intended application.
However, after recently digging into more details about the distinction between saturation and triode regions in MOSFETs, it occurred to me that it would make sense for the Rds(on) test voltage to be equal to (or at least close to) the typical Vgs at the boundary between those two regions, where Vds ≈ Vgs - Vgs(th). This would make sense because the majority of the variation in Rds occurs in the saturation region, and once Vgs has risen (or fallen, in the case of a PFET) enough to put the MOSFET into the triode region Rds is close to approaching its lower limit.
Is this generally the case? Do manufacturers pick an Rds(on) test voltage fairly close to the saturation/triode region boundary, or is it essentially arbitrary?