I can't imagine why the manual on a '97 would talk about a brake resistor unless they were just doing a "cut and paste" from a much older manual.
As I recall, from the dim memories of the past, the early brake controllers were sometimes factory set for a 4 brake system, and if used on a two brake single axle trailer you needed a brake resistor to cut the current down. Also, if your really old trailer had 6 volt brakes and you had a newer (at the time) 12 volt
system you used a resistor.
But all modern brake controllers that I am familiar with do all the work electronically and have enough range built into their adjustments to compensate for any combination of single to tri axel setups.
Bottom line: I would not worry about it. Just get a good modern brake controller, set it up by the directions provided by the brake controller manufacturer, and you will be good to go.