Unless you've custom ordered a different setup, the middle knob on your Z. S. Thomas guitar controls a blend pot.
The blend pot allows you to pan back and forth between the two audio sources that are your bridge and neck pickups. As the diagram below shows, when the knob is fully clockwise the neck pickups output is at 100% and the bridge is at 0%. Turning the knob to the middle "center detent"* position lets the signals from both pickups pass through uninhibited.
*The "center detent" is a notch you can feel when the knob reaches the center position.
The blend pot has a linear taper, which allows you easily adjust the output of the pickup you're adding into the mix. Turning the knob creates subtle differences in your sound, and gives you access to a larger tonal palate than that of a standard 3-way toggle switch.