Optical systems use data captured from camera image sensors to triangulate the 3D position of a subject between two or more cameras calibrated to provide overlapping images.
Data collection can be done by small dots (markers, passive or active) on an actor, or by recognizing and calibrating on the basic body parts of an actor.
With adding more camera's it is easy to expand the trackable area, the number of traceable subjects.
Optical tracking systems require a line of sight but can be used in an environment with magnetic disturbance. When it is impossible to maintain line of sight with the user in your setup, it might be best to switch to a magnetic tracking solution.