Let's keep this topic very simple.
Door opening is crucial, there is nothing special about it.
It's very obvious that having a wrong door circulation can cause many problems for people. For example... people with wheel chair, can they fit through? Can furniture fit through? Can office people move comfortably? There is more than what you think. I recommend you to always check your local standards to meet it's criteria. For example, in Australia we always refer back to AS1428 "Design for access and mobility" and Building Code of Australia (BCA).
Below is an image and a rough guide to understand the door opening.
Though rookie architects always makes a mistake (so did i) that the door opening is from the frame. Have a look at the image below it makes more sense
Door opening is always calculated from the inside face of the door panel when is open to the inside face of the door frame.
Keep in touch
TK