Can't find much:
A pole should be “planted” approx 6ft (1.8m) into the ground.
The doby mark is a notch 3 metres from the bottom of the pole. A pole then
correctly planted would show the 3 metre mark at a height of 1.2 metres
above ground level.

Often “Depthing Tubes” are fitted which allow inspectors to check that
those rascal erection contractors have buried them deep enough.