Which of the following are classification systems used for regulating UAS?

The classification systems mentioned in the answer focus on practical and operational characteristics that are crucial for regulatory purposes regarding unmanned aircraft systems (UAS). Gross takeoff weight (GTW) is particularly significant as it helps define categories of aircraft based on their mass, which is a primary factor influencing airspace usage and safety regulations. Kinetic energy is relevant in understanding the potential impact of a UAS during a collision, which is vital for safety assessments. The number of power plants and the size of the air vehicle contribute to understanding the complexity, performance, and operational limitations of a UAS, which aids in establishing appropriate regulations for their use in different airspace environments.

The other options primarily include characteristics that, while important in the operational context, do not serve as primary regulatory classification systems. Maximum speed and altitude capability may inform how and where a UAS can operate, but do not directly categorize UAS for regulatory purposes. Similarly, flight range and payload capacity, while significant for operational capability, do not fit within a structured regulatory classification framework as effectively as the elements listed in the chosen answer. The other options contain characteristics that are more operational or performance-based, rather than strictly classification criteria used for regulatory oversight.