When creating a custom status for an order, the inventory is either increase or decrease but sometimes we just don't want the inventory to be affected in anyway just to change the status of the order. There should be a no action for inventory.
Everything depends on the Inventory setting of the used order statuses.
For example, the inventory will be changed only if you change status with Inventory=Decrease to Inventory=Increase and vise versa.
If you change from Inventory=Decrease to Inventory=Decrease or Inventory=Increase to Inventory=Increase, the inventory will not be affected