"Windows 11 26H1" is a special version of Windows exclusively for new Arm PCs
Since its release in the fall of 2021, Microsoft's Windows 11 has received an "annual feature update" in the second half of every year. These feature updates sometimes include new Windows features and other changes that are too large to roll out in a typical monthly Windows Update, and users need to upgrade to new ones to keep getting security patches and other features. The currently supported versions are Windows 11 24H2 and 25H2, released in the fall of 2024 and 2025, respectively.
This week, Microsoft disrupted that update cadence by announcing more information on Windows 11 26H1, which is best described not as an update to Windows 11 but as another version of the operating system entirely. That's because 26H1 is a "scoped" release intended exclusively for new PCs, starting with those based on Qualcomm's recently announced Snapdragon X2 Elite chips.
Microsoft's support page explains why this release is strange: It won't be released broadly to other Windows 11 PCs, which should continue to use either 24H2 or 25H2. PCs running 24H2 or 25H2 will never be offered an update to version 26H1, though testers in the Windows Insider Program's early access Canary channel are able to install it to other PCs if they want. (Build numbers for Windows 11 26H1 start with 28000, compared to 26100 for 24H2 and 26200 for 25H2.)