Apple will release OS X 10.11 El Capitan on September 30 to all users for free. But, as that date approaches, it’s still issuing new iterations of the software to developers. The first is in the form of a brand new beta, OS X 10.11.1 beta 1, which is available for developers now.
Build 15B17c should be available either as an update through the App Store or through the Apple developer portal. According to AppleInsider, the focus is simply on “the stability, compatibility and security of your Mac.” That’s obviously going to be more and more important moving forward, especially now that Google is calling out companies that publish insecure software to customers.
This beta will probably trickle down as one of the first public updates to El Capitan sometime after September 30.