The Default Delivery Method selection criteria is used to indicate whether the resulting output should include invoices for projects that have a particular Invoice Delivery Method specified (as defined on the Project Profile >> Billing >> Invoice Setup >> Contacts tab >> Invoice Delivery Method field -- for the Lead Project on an Invoice).  You can choose to limit the resulting output to those having one, two or all three of the possible settings.   Leaving all three unchecked is equivalent to checking all three.

Regardless of the project's default invoice Delivery Method, the user can choose to email or print any invoice.