Cura G-code Printer G-code settings vs. Extruder G-code settings
If your printer has only one extruder, leave the Extruder G-Code empty. This setting is useful for multi-extruder printing (e.g. multicolor). The extruder G-Code runs each time you switch to and away from that extruder, and can be used for stuff such as retracting a filament when switching.