HMDs are classified into two types based on technology: augmented reality and virtual reality. Augmented reality HMDs overlay digital information onto the user's view of the real world, while virtual reality HMDs create a fully immersive virtual environment.HMDs are also used in various applications in the aerospace industry, including military, non-military, and commercial. Military HMDs are used for training, simulation, and combat operations. Non-military HMDs are utilized in search and rescue, disaster response, and other applications. Commercial HMDs are used in aviation, industrial, and entertainment applications.