They are built from checkouts whose shipping method and date match the day you picked — yesterday, today, tomorrow, the named upcoming days, or the Trigger Date for the 'on X Day' variants. Pickup Locations narrows pickup orders to one pickup point, also using the Trigger Date. For subscription orders, Sprwt only includes days that are still active (not skipped or cancelled by the customer); for a la carte, packs and other non-subscription order types, every checkout on that date is included. This is meant for day-of reminders such as "Your delivery arrives today between 12-2pm" — check the Recipients count (or a super-admin Preview) before confirming a large send.