The standard length against which all the length offsets are measured.
May be a normal tool, a token tool (e.g. a piece of drill rod), or may
even be an imaginary tool (e.g. the distance from Z home to the measuring
The position the Z axis moves to for tool changes, in response to the
M25 code, and when the TOOL CHECK key is pressed. This position is
set in the Z coordinate of Return Point #1 (the G28 position). This is
usually zero, making it the same as the Z axis Machine Zero position.
The Z axis position from which the reference tool touches the tool
measuring surface. Since the tool measuring surface is not necessarily
the part surface, Z Reference is not necessarily zero. The Offset Library
displays Z Reference in local coordinates, but it stores the value
internally in machine coordinates. Thus there is no need to reset Z
Reference after changing the part zero location, even though the displayed
value of Z Reference will appear to have changed.