It goes something like this....
Certain deliveries require signatures, where the actual driver must get a recorded signature from someone that accepts the package (say UPS delivering speakers). Some deliveries need a signature, but not by someone in person. So, you can leave a sticky pad on the door telling the delivery guy to leave it at the door, and then sign it (with teh name of the person the package is being sent to). If you leave the sticky note out and you get a note saying that a signature is required and the guy will back at a certain time, then you know what kind of delivery you have