Boot Camp Assistant determines your installation requirements based on the model of your Mac. To find out whether your Mac can perform a media-free Windows installation—or if it requires installation via an optical drive or a USB drive—open Boot Camp Assistant. It's in the Utilities folder inside your Applications folder.
Boot Camp Assistant shows you only the option that your Mac supports and guides you through the installation process.
If your Mac supports media-free installation, Boot Camp Assistant can perform the installation using a Windows disk image (ISO) downloaded to the hard drive on your Mac. Boot Camp Assistant also automatically installs Windows support software from Apple. With this type of installation, you only need a Windows ISO downloaded from Microsoft.
If your Mac supports USB installation, you need an external USB disk or USB flash drive that contains a Windows ISO. Learn more about installing Windows on your Mac using a USB hard drive or flash drive.
Optical drive installation
You need a Windows installation disc if Boot Camp Assistant presents the screen shown below. This screen doesn't offer the option to Create a Windows 7 or later version install disk on a USB flash drive. Boot Camp Assistant can download and install Windows support software from Apple. Learn about the steps to follow if you're installing Windows 7 or earlier.
Learn more about how to install Windows using Boot Camp.