This program defines a yard as any track or tracks in a town where cars are stored for other trains to pick up. (This may or may not be the definition of a yard as far as your prototype uses. For example, every town in the CPR was a "yard".)
This program can handle a number of yard types. This includes a normal yard, a Fiddle Yard and a Staging Yard (explained in the links). There is a fundamental difference between a normal yard, and a fiddle yard or staging yard. The main difference is that fiddle and staging yards have no corresponding prototype. They are solutions to modeling in a closed environment.
You must define one yard as the default yard. If you have a fiddle yard it automatically becomes the default yard. However, you cannot have a staging yard as the default yard. If you do not select the Staging Yard or Fiddle Yard options when you create a yard, it will be just a normal yard. Each of these yards can also perform one or two functions. A yard can be used to store empty cars. By default the Default yard and Fiddle yard must store empties, but a Staging yard cannot. Even though a staging yard cannot store empties, you can indicate that empties be sent to a staging yard to be returned as full cars.
Any other yard not set to store empties, which have empties arrive at them, will be automatically picked up and sent to the end yard of the train. Reasons for having yards for not storing empties includes transfer yards which as used only for trains to exchange cars at.
You also have the option to have a yard behave like an industry, which would allow you to make shipments to and from such yards. That is, any full cars from an industry that needs to leave the layout through a yard, will have that yard do shipments. When the full car arrives at this yard it will become empty and ready for another shipment. Conversely, shipments from a yard (off layout origin) to an industry on your layout, any suitable cars found to facilitate the shipment will be filled at the yard and put on a train for delivery. By default the Fiddle yard and Staging yard can make shipments. But you can have any additional yards perform this function.
There are a number of options for allowing a train to visit a yard and to switch a yard as an industry. Thus, there are a number of combinations with yards acting also as industries that are tabled below
The odd one in the table is #7. This allows you to have the train not visit the yard to drop off or pick up cars that are going to other places, but does allow you to use that yard as an industry. Thus you can still send shipments to that yard and drop off cars which will be emptied. You can also pick up cars that are doing a shipment from that yard.
In summary, a yard can be your normal expectation of what a yard is. But this program expands on that notion more than above. For example, if you have a junction town to a branch line, and cars are dropped off at this town for trains on the branch line, then you will have to have some track located somewhere where you drop those cars off at. That one length of track would be a "yard" in this case. If you have freights that drop cars on a siding in a town for another train to do the switching, that length of track would be a yard.
In the demo you will see 10 yards which show these various types. The yards at Palmerston, Orangeville and Owen Sound one would call normal yards. They store cars for various trains, and they store empties, but they do not make shipments. Lambton yard is the Default yard, as well as being a Fiddle yard. Most of the shipments on this demo go through Lambton yard. There is also a yard in Saugeen Junction. Actually, all the CPR had at Saugeen Junction was one track. That track was the yard (I have the yard limit sign for Saugeen Junction). This yard allows cars to enter the Walkerton Branchline as no train on that line leaves that line. Two terminal yards, one at Walkerton and the other at Teeswater, are required because trains stayed over night and cars will have to be stored. Trains must originate from yards. Again, this was only one track at Teeswater used to store cars. Guelph Junction is a Staging yard used to bridge traffic on the main line to Lambton.