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Glossary

Height Offset: The difference in length between a tool and some standard length. Used interchangeably with Length Offset.
Length Offset: The difference in length between a tool and some standard length. Used interchangeably with Height Offset.
Reference Tool: 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 surface).
Z axis Machine Zero: The position the Z axis moves to during the automatic homing sequence after power up. Zero in machine coordinates.
Z axis Part Zero: Usually the part surface. Specifically, the position of the Z axis when the selected tool is touching the part surface. Zero in local (workpiece) coordinates.
Z Home: 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.
Z Reference: 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.