One way to specify them for the benefit of operating crews is to provide town maps (or in later eras, SPINS documents or equivalent). I alluded to this usage in my column in Model Railroad Hobbyist that was about timetable construction. That column was in the issue of MRH for October 2014, and like all issues of MRH, it can be downloaded for free by anyone at the MRH website, www.mrhmag.com . Here is an example of one of my town maps, for the switching district of East Shumala (click to enlarge).
Above, you see that the Phelan & Taylor packing house at East Shumala has three spots identified. Note the instruction, inbound freight to door 1; but all doors can be worked for outbound produce loads. The actual model industry looks like this in a somewhat aerial view:
The storage shed at right, with its door equally spaced (for 40-foot reefers) with the two doors on the main building, is only used during high shipping season. Thus empty reefers are normally spotted at doors 1 and 2. The foreground stock pen is a single-spot example.
Many of my industries have a single door or a single spot, and don’t require any specified sure spot. A good example is Pismo Marine Service, a ship chandler, at Santa Rosalia. There is only one door and one short loading dock, and clearly only one car spot. The building also hosts Martinez & Son marine engine service and repair.
Some spots are not immediately obvious. My ice deck is supplied from an ice house which is primarily a storage house, and the only ice produced here is consumer ice. To be able to ice reefers with thousands of pounds of ice per car, the facility receives ice shipped in. This shipping is accomplished with PFE Ice Service cars, and they have to be spotted at the receiving door. That door is shown in this early-morning view, on the side of the ice house.
Often during an operating session, one of my Ice Service cars will stand here throughout the session, but from time to time, an empty Ice Service car is to be picked up, or else an inbound load of ice has to be spotted at the receiving door. Cars being iced, of course, are spotted alongside the deck, not at the receiving door. Here is a photo of an ice delivery car on spot.
The variety of switching requirements with sure spots adds complexity and thus interest to operating sessions, as well as reflecting a range of prototype needs and arrangements. I have assigned sure-spot designations for all the industries on my layout for which they make sense.