If you choose this mode, the customer will be able to select a shipping date, and all products will be shipped together on that specific day. That day will be limited to the latest preorder date available, for instance, if the order has 3 different products marked as preorder ones:
Then, the minimum shipping date will be the 1st of October (i.e: the minimum date when all products will be available).
If you choose this mode, the customer will get two different orders generated, one for preorder products and one for in-stock ones. For instance, if your order looks like this:
Then you will get an order which will be processed now, with products A and C, and then another order which will be shipped on the 1st of Septemb
If you choose this mode, then the customer will get one order for all in-stock products, and then one order for each preorder product, which will be shipped in each specific preorder date.
Use this mode if you want to only allow your customers to either choose preorder products or available ones.