How does MPEG-2 defines mux rate?

How does MPEG-2 defines mux rate?

MPEG-2 defines mux rate as the combined rate of all video and audio elementary stream packets common to one program. This rate also includes the VBI and sub-picture private stream data, which MPEG treats as a private stream type. Mux rate is always specified as 10.08 Mbps because this is the rate at which user data arrives into the track buffer.