HEIDENHAIN ND 920 - Manual (Page 7)

7
Nominal position, actual position and distance-to-go
The position to which the tool is to move is called the nominal position
( S ). The position at which the tool is actually located at any given
moment is called the actual position ( I ).
The distance from the nominal position to the actual position is called
the distance-to-go ( R ).
Sign for distance-to-go
When you are using the distance-to-go display, the nominal position
becomes the relative datum (display value 0). The distance-to-go is
therefore negative when you move in the positive axis direction, and
positive when you move in the negative axis direction.
Fundamentals
Y
X
Z
I
S
R