From New Telemark Guide

Boot center, in this case telemark boot center specifically, is the midpoint between end of the heel and the 3-pin line on a telemark ski boot.

It is commonly used to align the skier's foot to a mounting reference point along the longitudinal axis of the ski. In contrast to using the 3-pin line of the boot, the boot center convention abstracts the boot size from the alignment. Boot center reference for a small foot is the same as for a large foot.

NTN boots have a molded boot center mark like an alpine boot. Most 75mm boots do not, and the mark must be measured and marked manually.