Real IPD is the actual distance between the center of your eyes' pupils in real life. Any Head-mounted Display require this measurement to accurately position the virtual camera outputs on the screen in front of the user's eyes. The outputs have to be directly in front of both of the users' eyes. Any deviation can cause eye strain. Most HMDs allow users to adjust their camera output positions to match their IPD.
Additionally, some HMDs allow the physical position of the lenses and screens to be adjusted to match the user's physical IPD.
The average IPD for humans is 64 millimeter, with a range of 54 to 72 millimeter. You can measure your IPD by measuring the distance between the center of your pupils with a ruler.
Virtual IPD, sometimes referred to as ICD (Inter Camera Distance) or VCS (Virtual Camera Separation), is the distance between the 2 virtual eyes (cameras) in Virtual Reality. This number can change the Sense of Scale of the objects in the virtual environment in relation to the user.
Developers can change the sense of scale by changing virtual IPD. They can cause the environment to increase or decrease in size by changing the virtual IPD.