What does the Fixed ES Rate checkbox mean in the MPEG Player tool?

What does the Fixed ES Rate checkbox mean in the MPEG Player tool?

The fixed ES rate checkbox causes the elementary stream to be played out at a fixed rate regardless of the clock rate. This essentially would cause a stream to be stuffed with null packets or be missing data rather than be played too fast or too slow.

The clock screen is covered in more detail on pages 3-8 and 3-9 of the MTX100A user manual.