The scheduled shipping date is based on the request date as long as the setting for 'Ship based on conf date' is not activated for the specific order.
More information is found here: Sales order - COA
Specifies when the goods should be shipped. This is used to calculate the scheduled shipping date for each item.
Ship based on confirmed delivery date:
If enabled, the scheduled shipping date for each item is calculated based on confirmed date rather than the requested date.