HEIDENHAIN TNC 360 ISO Programming - Manual (Page 74)

TNC 360 4-10
4 Programming
4.2 Tools
Tool change position
A tool change position must lie next to or above the workpiece to prevent
tool collision. With the miscellaneous functions M91 and M92 (see page 5-
39) you can enter machine-referenced rather than workpiece-
referenced coordinates for the tool change position.
If T0 is programmed before the first tool call, the TNC moves the spindle
to an uncompensated position.
If a positive length compensation value was in effect before T0, the clearance to the workpiece is reduced.