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from wikipedia

Eye trackers necessarily measure the rotation of the eye with
respect to the measuring system. If the measuring system is head
mounted, as with EOG, then eye-in-head angles are measured. If the
measuring system is table mounted, as with scleral search coils or
table mounted camera (“remote”) systems, then gaze angles are measured.

In many applications, the head position is fixed using a bite bar, a
forehead support or something similar, so that eye position and gaze
are the same. In other cases, the head is free to move, and head
movements are measured with systems such as magnetic or video based
head trackers.

For head-mounted trackers, head position and direction are added to
eye-in-head direction to determine gaze direction. For table-mounted
systems, such as search coils, head direction is subtracted from gaze
direction to determine eye-in-head position.

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