A deletion of a STO does not make the requirement go away. You have to reject the sales order to eliminate the requirement.
the fixed indicator can be set automatically for requisitions based on customizing and is automatically set when you do a manual change. If the fix indicator is active, then the requisition is not changed by an MRP run.
e.g. if you have a sales order but no stock, then MRP creates a requisition. if you then do a physical count and found stock, the next MRP run would remove a requisition which has no fixed indicator as it is no longer needed to procure this part. If the requisition is fixed, then MRP run will not touch this requisition after you found stock. So you finally have much more stock as you need.
It is up to you and your design if you want fix requisitions or not