Tool length compensation is what ever you set it to ( Tool Offset ) which you can adjust at any time, just as you would for tool wear on diameter when using X and Y Cutter comp
I want to make it clear so you stop wasting your time trying to get cutter comp to work in the Z axis, Cutter comp does not work on a Z axis it only works in X or Y axis so if you apply cutter comp when you are using a G18 it can only be applied to the X axis so has to be in the right place in the program for it to work, if anyone is telling you any different then they are just Bs'ing you Here is a good presentation of how to use cutter comp Note that Z axis can not be used to turn on or off cutter comp
https://www.cnc.com/g40-g41-g42-cutt...sation-gcodes/
You have to use a G141 for 3D cutter comp and I doubt that your control can use a G141