Skirt/Brim Extruder
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When printing a skirt or a brim, this setting determines with which extruder the main body of that will get printed.

The skirt and the brim will both get encircled with a ring for each extruder that is used in the print, to ensure that the material is primed. This setting only chooses the extruder to use for the first part of the skirt or brim.

Note that if the printer has multiple extruders, but the first extruder is not used in the rest of the print, this setting doesn't automatically adjust itself to one of the used extruders. Your print would then still use that first extruder, only for the skirt or brim. That might waste some time and material, or leave the wrong colour attached to the bottom of the print.