If D is not available it books into L (cheap economy booking class), with the main exception for AA 2-class north america flights which book into A (discounted first), and a few specific exceptions (eg certain airlines with 1 class, or where the highest class is premium economy). If L is also not available then you either have to open date the segment or change dates or routes.

Of course you could waitlist for availability to open up if you are happy to not be ticketed (has it's own risks eg lack of protection for rule changes or fare changes).