Figuring out where freight cars should go...
Each car can belong to a division which is used to identify where the empty car is supposed to go, as well as which industries prefer that car.
Divisions are completely optional; SwitchList should work just fine if you never fill in the Division fields in industries, or the Home Division field in freight cars. Divisions can be as simple as "Here" and "Elsewhere" to indicate which boxcars are for the local railroad and which are for everyone else, or could indicate the owning railroad. I usually treat the division as either a specific railroad or region of the country.
Divisions give you a way to restrict what cars get used. For example, bagged cement going from a cement plant on your layout (in the "SP" division) to an offline west-coast industry also in the "SP" division would only get assigned to cars in the division of one of the industries. An "East Coast" car shouldn't be used for this shipment, but canned fruit going to an offline industry in the "East Coast" division is appropriate.
Sample Layouts to see how divisions are used.