Z offset defines the relative position of the composite nozzle with respect to the plastic nozzle about Z-axis.
If the composite nozzle tip scratches the buildplate or previous layers, or the quality of vertical walls of your model is insufficient due to improper plastic layup, go through this procedure. Also, it is recommended to go through this procedure every time you replace the composite or/and the plastic nozzle.
On the screen of the printer, press Maintenance > Calibrate > Printhead > Comp. Z offset. and go through the built-in calibration procedure by following the instructions on the screen.