Double Drop Trailers are a specialized flatbed trailer and are generally used for oversize freight, and is a type of RGN or Removable Gooseneck Trailer. They can also sometimes fall into the Heavy Haul Freight category. However, they are often used for Flatbed freight as well, since they have so much deck space to work with.
A lower deck height allows you to transport higher equipment or material, and also utilizes a “well”, which allows freight like construction equipment, farm vehicles, and other things like this to sit in the middle. A front and back deck also allow for other partials to be loaded on as well.