Calibration is necessary to associate tracked VR devices and skeletal bones of a character’s 3D-model. The tracking of four bones is obligatory. These are the head, both wrists, and the pelvis bone.
The calibration window becomes available only after selecting skeletal mesh.
When making a new preset:
In case the straps of Vive Trackers allow you to remove them and put them on again while preserving their positions on the body, later you will be able to restore saved calibration without repeating steps 1-3.