Starting today, Apple is making the new macOS Sierra operating system available as an automatic download to customers running OS X El Capitan in order to encourage them to update.
Customers who have auto downloads enabled will see macOS Sierra start to download automatically, but it will not install without express user permission.
Apple shared the news with The Loop
and says the update will only be downloaded on computers that meet the specs for macOS Sierra and have an adequate amount of storage space.
Apple is also being smart about the download. If your computer is low on space, macOS Sierra will not download. In addition, if it has downloaded and your computer starts to get low on space, the download will be automatically deleted.
Customers who do not want to upgrade to macOS Sierra can manually delete the update to keep from being prompted to do the install, and the download can be turned off by disabling automatic downloads. Disabling the feature can be done by going to System Preferences -> App Store and unchecking "Download newly available updates in the background."
Automatic downloads of macOS Sierra will be rolling out to customers over the next week.