I'm not sure I understand this question.
You have
- the requested delivery date; this is the date that the customer requests goods to arrive at their plant
- the confirmed delivery date; this is the date that you confirm to customer you can provide delivery by
The requested delivery date will only be changed if you do it manually (it is usually not wise to change this date as it is the customer's requested delivery date, good business would strive to meet this).
For the confirmed delivery date, this gets calculated based on your scheduling and availability situation. If you want to ensure that subsequent availability checks do not change this date, then you set the "Fixed date & quantity" flag.