Very soon, both iOS 9 and OS X El Capitan will become available as public betas under the Apple Beta Software Program, which you can sign up for here. But to increase openness and expand its pool of beta testers, Apple decided to offer “public betas” of both iOS and OS X to interested users, starting with iOS 8.4 and OS X Yosemite.
Up until recently, unfinished “beta” versions of iOS and OS X were only officially available to registered Apple developers, enabling companies to make their apps compatible before the operating systems were publicly released.
