Windows 11 uses three different materials—Acrylic, Mica, and Smoke—to create its user interface (UI) and the graphical effects for its apps. Twitter user and leaker FireCube discovered that Microsoft had secretly introduced new content labeled “Mica Alt.”
— FireCube (@FireCubeStudios) September 6, 2022
FireCube has pointed out, and Microsoft’s own guideline page seems to agree, that the new Mica Alt content is designed primarily for Tabbed instances. In its App layering documentation, Microsoft addresses this issue:
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.
Mica Alt can be used “in apps that use Windows App SDK 1.1 or higher,” according to Microsoft’s stated requirements.