The visual effects in the Windows 11 user interface (UI) and its various apps are provided by three materials namely Acrylic, Mica and Smoke. Microsoft has now added one more material dubbed "Mica Alt" recently, which was spotted by Twitter user and leakster FireCube.
As noted by FireCube, the new Mica Alt material is generally meant to be used for Tabbed instances and Microsoft's guidance document sort of confirms it too. Microsoft talks about this in its App layering section of the guidance:
App layering with Mica Alt
Mica Alt is an alternative to Mica as a foundation layer in your app's hierarchy with the same features like inactive and active states and subtle personalization. We encourage you to apply Mica Alt as the base layer of your app when requiring contrast between title bar elements and the commanding areas of your app (e.g. navigation, menus).
A common scenario for using Mica Alt is when you are creating an application with a tabbed title bar.
In terms of requirements, Microsoft says that the new Mica Alt material can be used "in apps that use Windows App SDK 1.1 or higher".