Assuming Apple continues its previous practices, once macOS 10.13 High Sierra comes out, it will become impossible to download 10.12 Sierra for the first time. That could be awkward if you want to upgrade an older Mac to Sierra at any point after High Sierra ships, since you won’t be able to get Sierra then. To ensure that you can snag a copy of Sierra in the future, open the App Store app on your Mac, type Sierra in the Search field, and click the Get button for macOS Sierra (it’s about 5 GB in size). It downloads to your Applications folder, and the installer launches automatically. If you don’t want to install Sierra right away, choose Install macOS > Quit Install macOS to quit the installer. What’s important is that Sierra is now registered to your Mac App Store account, and you can get it again from the App Store’s Purchased screen at any time on any of your Macs.