Hey Tom,
M6 will stop the spindle and prompt the user to change the tool based on the last T- number programmed.
You may or may not have programmed in your CNC controller a position to which the spindle is moved for the manual tool change.
T (Select tool) – (optional)
Tx - prepare to change to tool x.
The tool is not changed until an M6 is programmed (see Section M6). The T word may appear on the same line as the M6 or on a previous line. It is OK if T words appear on two or more lines with no tool change. Only the the most recent T word will take effect at the next tool change.