The difference is the flange focal distance (the distance from the lens mount surface of the camera to the image sensor).

The specifications of the C-mount
Inner diameter: 25.4 mm
Thread pitch: 0.794 mm
Flange focal distance: 17.526 mm

The specifications of the CS-mount
Inner diameter: 25.4 mm
Thread pitch: 0.794 mm
Flange focal distance: 12.5 mm

To use a C-mount lens on a CS-mount camera, a 5 mm adapter ring is required.
A CS-mount lens cannot be used on a C-mount camera because it will not reach focus.

When selecting a lens, please make sure to check the camera’s lens mount type.