Virtual Reality (VR) technology is to create a lot of information to deceive human perception and brain, and make the user feel like being physically present in a non-physical world. The non-physical world can be a game, a concert, or a training program. Flight simulator for pilots training is a classic application of VR. The pilots practice in light simulator, so that they do not make real mistakes in airplanes. By the same concept, an electronic controller (the "brain" of a system) can be deceived and trained by a kind of simulator in a completely safe and repeatable environment. That is Hardware-in-the-loop (HIL), a mixed reality (MR, a combination of VR and AR) application in engineering.
By connecting to the physical system modeling, the HIL easily steps forward to the minimum viable Digital Twin, where the entire system is duplicated in the digital world.