I'm a newbie of iOS development and I'm confused regarding universal app.
We can use the same code, same xib file for iPhone 4(retina) and previous iPhones, but why we should write two different xibs for iPhone and iPad? What's the difference?
For iPhone and retina iPhone, we use "point" instead of pixel for the coordinate. Why we can't use the similar method for iPhone and iPad?
For some simple apps, it is possible to design your iPhone UI and reuse the same xib file for the iPad. Just select your Target in XCode and copy the Main Interface text from iPhone / iPod Deployment Info to iPad Deployment Info. If you're using a Main Storyboard, copy that too. However, the iPad does not simply scale everything up from the 320*480 / 640*960 iPhone screen to the 768*1024 / 1536*2048 iPad screen. @elgarva correctly says that this would look terrible. Instead, the iPad version makes use of your autosizing masks to resize or reposition each view.
If all of your views can be considered to be left-middle-right or top-middle-bottom, this may work. If you have anything more complicated, you'll need to design a separate iPad interface.
Duplicating your iPhone UI is not just discouraged for aesthetic reasons - iPhones often end up containing a deep and confusing navigation tree for tasks that the iPad can fit on a single screen.