Looking for a reliable diesel truck

Please help me understand. I have a 1999 Dodge diesel 2500, Long bed, auto trans, cub cab, 2 wheel drive. The owner manual on page 148 says that this truck can tow maximum trailer weight of 10,200 with 3.54 axle ratio and 12,000 lbs maximum trailer with 4.10 axle ratio. The GCWR at the highest is 18,000.

I can't understand how you can operate/tow this unit safety and not tear up your truck? Most everyone on this forum and others said I can only haul a trailer around 9,000 lbs. "Dry Weight" safety and do not max out my truck!