Where is the world coordinate frame referenced to in the function triangulate when two camera Matrices are provided?

I am using triangulate to get 3D points clouds from a stereo pair: [worldPoints_t,reprojectionErrors_t] = triangulate(matchedPointsL_t,matchedPointsR_t,PL',PR'); The triangulate documentation says that the function, "returns the 3-D locations of the matching pairs in a world coordinate system. These locations are defined by camera projection matrices." I am wondering if the world coordinate frame is aligned with the optical center of camera 1 (the left camera, PL' in my case) or the second?