The extra unretract length is the value additional to retract length. On print resuming, if portion of plastic has spilled from an extruder, then volume of plastic in a nozzle becomes less than what it was before travel. If you have noticed for this plastic (especcially for runny) that at the beginning of print there is an underextrusion migrating to normal extrusion, then you shoud set not null for this parameter.
