The links below are for 2021 F-350/450 pickup trucks (including box delete pickup trucks) and F-350/450/550 chassis cabs, but I think the information applies to 2017+.
From Ford's link about Super Duty
pickup trucks:
"The
fully boxed frame, built of percent high-strength steel with up to 10 crossmembers, is 24 times stiffer than the previous design. The frame is designed to minimize flexing and twisting while maximizing strength and torsional rigidity, and serves as the foundation for the impressive towing and hauling capability of the Super Duty® ." See:
From Ford's link about chassis cab trucks:
"The
chassis cabs have a fully boxed frame under-cab with C -channel aft-of-cab frame." See:
The Super Duty® Chassis Cab - Made of high-strength, military grade, aluminum alloy making it is light but incredibly strong. Check out more durable features.
www.ford.com
Some chassis cabs can be ordered with aft-axle frame extensions. These can be useful for supporting longer boxes while keeping the wheelbase short for an improved turning radius (compared to a longer wheelbase for the same length box).
Flatbeds are available for pickup trucks and chassis cabs, but, because of the difference in frame shapes, aren't interchangeable.