XR is generally used as an umbrella term and is frequently used as a casual shorthand to group technologies such as VR, AR and MR together.

Here the 'X' can be seen as a placeholder for V(R), A(R) or M(R)

Mixed Reality, Merged Reality and Digital Reality are one and the same.

XR also covers the hardware, software, related technologies, methods, and experience.

Physically Based Rendering (PBR) is an approach in computer graphics that seeks to render graphics in a way that more accurately models the flow of light in the real world.

PBR approach is originally explored by Disney and adopted for real-time display by Epic Games. Later, widely adopted on most popular engines and looks visually amazing.

Each of the surface parameters we need for a PBR pipeline can be defined or modeled by textures like Albedo or Diffuse Map, Normal Map,Roughness Map, Metallic Map and Ambient Occlusion Map.