The coordinate system from the return of from_file_multisweep()

Can someone tell me which coordinate system is the returned point cloud of “from_file_multisweep()” on? Is it on the coordinate system of a lidar reference frame when aggregating radar sweeps as the comment in source code shows ? As I see the following transformation, but I’m not sure about it. It would be very helpful if you can explain to me a little bit more detail about this transformation process. Many thanks in advance!

trans_matrix = reduce(, [ref_from_car, car_from_global, global_from_car, car_from_current])

(link: source code)