On the TDS200 series what's the little arrow at the top of the graticule?

On the TDS200 series what's the little arrow at the top of the graticule?

It indicates horizontal position (i.e. trigger position) and is coupled with a numeric readout above the graticule:

0.00 = center screen (mid record).

Horizontal position is read out in + or – time with respect to the center screen.