The “Thumb Region” is the highest fingerboard region, starting approximately from 1st finger on B on the “A” string (and its equivalent on the other strings). From here on upwards, our hand is so high up the fingerboard that the thumb can no longer reach under (behind) the cello neck, and therefore absolutely must be brought up on top of the fingerboard.
“THUMB POSITION” AND “THUMB REGION”: TWO DIFFERENT CONCEPTS
Whereas we use “Neck Position” hand postures in the “Neck Region“, and “Intermediate Position” hand postures in the “Intermediate Region”, the “Thumb Position” hand posture can be used in all the fingerboard regions. This is because we often bring the thumb up onto the fingerboard earlier (in the Intermediate and Neck Regions), not because we have to, but rather because we want to, in order to make use of it as an extra finger. Sometimes it can be useful to differentiate between these three different thumb position zones (Neck, Intermediate, and High).
The whole enormous subject of Thumb Position – in all the three fingerboard regions – is dealt with in its own section (click on the highlighted link).