What is the Horizontal position reference to?

What is the Horizontal position reference to?

The Horizontal position value (located below the graticule area, to the right of the Time/Div setting) is the time between Time Zero (the first trigger point) and the Horizontal Reference Point.

To verify this, use the vertical cursors and put one of them on the Horizontal Reference Point. The readout for that cursor will match, the Horizontal position value.