Table 1.

Classification of the level of immersion in virtual reality (VR) systems [18-21], the types of head-mounted displays, and the VR content.

Variable			Definition		Hardware/examples
Immersion
	High (immersive VRa)	The user is immersed within a 3D content.		HMDsb
	Medium (semi-immersive VR)	Rooms within which computer-generated environments are projected onto the walls.		C-Automatic Virtual Environment (CAVE)
	Low (desktop VR)	Computer-generated environments made in 3D but which are shown on 2D displays.		PC, television, mobile phone, or tablet screen
Types of HMDs
	PC-based	Requires a connection between an HMD and a computer with advanced computational and graphics capabilities.		Oculus Rift S and HTC Vive
	Console-based	Needs a connection between an HMD and a specific game console.		PlayStation VR
	Mobile	Involves the integration of VR systems on mobile devices thanks to specific HMDs.		Samsung Gear VR and low-cost HMDs compatible with mobile phones, such as Google Cardboard
	Standalone (all-in-one)	They do not need other technologies to work.		Meta Quest II, HTC Vive Focus, and Pico Interactive Neo
Content
	Virtual environments	Simulation on a computer of 3D environments, which can be explored in real-time and in which the user can interact with objects contained within it [59]. They are created through specific software such as Unreal or Unity.		N/Ac
	360° videos and photos	Images or videos of real or digital environments presented in a spherical version. They provide a spherical view with multiple viewing angles and perspectives. The contents are omnidirectional and can either be computer generated or captured from the real world [60].		YouTube VR and Google Earth VR
	Embodied virtual agents	Graphical representations of the individuals controlled by the computer itself using an artificial intelligence program [61].		N/A
	VR video games	Video games played through HMDs in which the player can interact with virtual content not only through a joypad or a keyboard, but also using head rotation, eye movements, or specially designed controllers that respond to the position and movements of the player in a defined space [62]. They include the following: - Commercial off-the-shelf video games that are “games that one can purchase on the high street” [63], or rather purchasable in online or physical stores - Custom-made games, often defined in the literature as “serious games” [64], that are games created ad hoc by researchers to educate, train, or change behavior		Beat Saber, Half-Life: Alyx, Superhot VR, and Fruit Ninja VR
	Social VR platforms	3D virtual spaces where multiple geographically remote users can interact with one another through VR HMDs [65,66]. At present, several online VR applications with a social component exist.		AltspaceVR, Horizon Worlds, VRChat, and VRzone

^aVR: virtual reality.

^bHMD: head-mounted display.

^cN/A: not applicable.