According to the release notes. iTunes 12.9 adds support for iOS 9 and is designed to work with OS X El Capitan, Apple next version of OS X, which will be released on September 30.
Apple says that iTunes 12.3 also:
Roman Loyola
Apple releases iTunes 12.3 update
16.09.2015
While you’re busy upgrading to iOS 9 (or trying to decide if you should upgrade in the first place) you can also update iTunes. Apple on Wednesday released iTunes 12.3, which you can update through the App Store.