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The Factory project is the rolling development codebase for openSUSE Tumbleweed.
There is a constant flow of packages going into Factory. There is no freeze; therefore, the Factory repository is not guaranteed to be fully stable and is not made available to download by humans. The core system packages receive automated testing via openQA.
When automated testing is completed and the repo is in a consistent state, the repo is synced to the download mirrors and published as openSUSE Tumbleweed. That usually happens several times a week.
Factory is mainly used as an internal term for openSUSE's distribution developers, and the target project for all contributions to openSUSE's main codebase.
Power users, developers, and openSUSE contributors are recommended to use the Tumbleweed rolling release.
Conservative users who just need a working system ("if it ain't broke, don't fix it") are advised to stay with the Leap stable release.