[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
public class ExportAvaloniaModuleAttribute : Attribute

Avalonia Modules can either be platform independent (ex default control styles provider) or dependent on a specific windowing or rendering subsystem being used (ex native rendering speedup, subsystem-specific interop backends). In the case of a subsystem-specific module, you can specify multiple module implementations, and also a fallback platform-independent module if you so choose. Additionally, these different implementations can be in different assemblies. They just need to all share the same module name. For example, if I had a module Foo that has a special back-end for Skia and a less performant/less user friendly back-end for any other rendering subsystem, I would do the following:

// In assembly FooModuleSkia.dll
[assembly:ExportAvaloniaModule("Foo", typeof(FooModuleSkia), ForRenderingSubsystem="Skia")]

class FooModuleSkia
{
    public FooModuleSkia()
    {
        InitializeModule();
    }
}

// In assembly FooModuleFallback.dll
[assembly:ExportAvaloniaModule("Foo", typeof(FooModuleFallback))]

class FooModuleFallback
{
    public FooModuleFallback()
    {
        InitializeModule();
    }
}

The fallback module will only be initialized if the Skia-specific module is not applicable.