The National Wildfire Coordinating Group establishes the standards to describe the capabilities of firefighting aircraft, crews, and equipment. For example, a Very Large Air Tanker must be able to carry at least 8,000 gallons of water or retardant, while a Type 4 air tanker carries less than 800. This makes it possible to order a resource that fits a specific need.
The chart above is the latest released by the NWCG. The only significant changes I could detect are the addition of examples of resources and a new category of aircraft, Unmanned Aerial Systems (UAS).
With the increasing use of UAS, or drones, the organization felt the need to set up standards for these aircraft that have a wide variety of designs. The new requirements specify endurance, altitude for data collection, and maximum range. Surprisingly the NWCG did not establish a requirement for payload capacity. In the foreseeable future drones will be delivering supplies to firefighters in remote areas. It will be necessary to set up categories for aircraft that can carry 25 pounds, 6,000 pounds, or anything in between.