Prior to El Capitan, you had to insert a USB Flash Drive and Boot Camp Assistant copied the Windows installer from an ISO disk image to the flash drive, and then downloaded and set up the Windows drivers to the correct location in the installer for the Mac hardware. Notice the ISO image and partitioning are all on a single screen. When you open Boot Camp Assistant on a new Mac that supports Windows 8 or later, you’ll get the new Boot Camp interface. Some very interesting changes in how Windows is installed in Boot Camp on OS X 10.11 “El Capitan”.
