Thank you

Adding oil changes the engagement point of the fork spring perch and outer chamber fork oil. The fork spring perch would engage the oil in the outer chamber sooner and start providing additional compression resistance sooner. Often times this makes the fork have a harsh “mid stroke” if the oil volume is near the upper limits of the oil volume parameters because of how soon / early the spring perch engages the oil. For some setups this is advantageous, for other setups it is not. The volume spacer is another way to modify internal fork volume without changing the oil volume and the spring perch engagement point.