This is a library that is directly referenced by an application. In some applications, there will be direct dependencies such as Wi-Fi or Bluetooth libraries. You may also have changed the location or version of an indirect library, which makes that library a direct dependency.
You manage the .mtb files for direct dependencies, and .mtb files should be checked into source control along with your application source code. A .mtb file for a direct dependency is typically stored in the application’s deps subdirectory by default. You can store it anywhere inside the application except in the application's libs subdirectory.