'special instructions' notification varies from carrier to carrier, and any other handlers inbetween, as to how the deliverer is informed (label, paperwork, phone or computer). as with most things, the fewer the handlers, the better.
specific 'special instructions' can be anything which would enable a successful delivery: a general time of day for delivery, an alternate delivery location, a warning about the fragility/nature of the goods or the nature of the delivery location (mind the dog, cows on road, sound horn at the gate, address hidden from road, etc). i even once specified the deliverer to 'knock hard, and he duly did. and prior to internetted mobile phones becoming all the rage (with their e-mail notifications of expected delivery), you could specify that the deliverer phone you before attempting delivery (some carriers do this as standard, now, especially with large items).