Hi Antek! Fabric team member here. Great question--though I'm not a lawyer, I can try to clarify Fabric's licensing a bit, and am happy to follow up with our legal team if you have any more questions.

Because Fabric is a mix of MIT-licensed open source code as well as branded assets--such as fonts, icons, and product logos--there's some nuance to what kinds of projects the license currently covers. Essentially, Fabric's assets can only be used today to design and develop apps or services that integrate with Microsoft products. These include Add-ins for Office, web parts for SharePoint, and other extensions for Microsoft products. This applies to usage of the assets in application code itself as well as any designs produced by the toolkit.

That said, all of Fabric's code itself can 100% be used in open source apps. There's even first-class support for Selawik, an open source drop-in substitute for the Segoe UI font used throughout Fabric.

Hope this helps. Again, I'm happy to follow up with our friends in brand & legal if you have more specific questions about Fabric's licensing or usage.