Update README with current implementation details

Author: guimafelipe

Samples/MultiHeadedPackage/README.md

@@ -23,16 +23,22 @@ These samples demonstrate how to create **multi-headed packages** — MSIX or sp
 A WinUI 3 solution using single-project MSIX packaging with two projects:
 
 - **PrimaryApp** — The main WinUI app that owns the package manifest. Uses `$targetnametoken$` tokens so the build system fills in the correct executable name.
-- **SecondaryApp** — A secondary WinUI app referenced by PrimaryApp. Its executable name is hardcoded in the manifest as `SecondaryApp.exe` with `EntryPoint="Windows.FullTrustApplication"`.
+- **SecondaryApp** — A console app included in the same MSIX package. Its executable name is hardcoded in the manifest as `SecondaryApp.exe` with `EntryPoint="Windows.FullTrustApplication"`.
 
 After deployment, both apps appear in the Start menu as separate entries.
 
 ### Sparse sample (`cs/cs-wpf-sparse/`)
 
 A WPF solution demonstrating multi-headed sparse packages with runtime registration:
 
-- **PrimaryApp** — A WPF app with `WindowsPackageType=Sparse` that includes Register/Unregister/Restart buttons to manage the sparse package at runtime.
-- **SecondaryApp** — A minimal WPF app that detects whether it has package identity (which it will once the sparse package is registered).
+- **PrimaryApp** — A WPF app with `WindowsPackageType=Sparse` that registers the sparse package at runtime using the `PackageManager` API. After registration, it uses `IApplicationActivationManager` to relaunch with package identity via the app's AUMID (Application User Model ID).
+- **SecondaryApp** — A minimal WPF app that detects whether it has package identity. Once the sparse package is registered, the primary app can launch this secondary app with identity using the `SecondaryApp` AUMID entry point.
+
+Key implementation details:
+- The manifest uses `uap10:AllowExternalContent="true"` to enable sparse packaging.
+- Application entries use `uap10:RuntimeBehavior="win32App"` instead of `EntryPoint` (these are mutually exclusive).
+- Restart/launch uses `IApplicationActivationManager.ActivateApplication()` with the package AUMID to ensure the process receives package identity. Launching the exe directly with `Process.Start` would bypass the package activation and the process would not have identity.
+- A test certificate (`PrimaryApp_TemporaryKey.pfx`) is included because sparse packages must be signed to be registered.
 
 ## Prerequisites
 
@@ -52,14 +58,17 @@ A WPF solution demonstrating multi-headed sparse packages with runtime registrat
 
 1. Open `cs/cs-wpf-sparse/MultiHeadedSparse.sln` in Visual Studio.
 2. Set **PrimaryApp** as the startup project.
-3. Press **Ctrl+F5** to run without debugging.
-4. Click **Register Package** to register the sparse package.
-5. Click **Restart** to relaunch with package identity.
-6. Run **SecondaryApp** separately to verify it also detects the package identity.
+3. Trust the test certificate: run `certutil -addstore TrustedPeople cs\cs-wpf-sparse\PrimaryApp\PrimaryApp_TemporaryKey.pfx` from an elevated terminal.
+4. Press **Ctrl+F5** to build and run without debugging.
+5. Click **Register Package** to register the sparse package with the OS.
+6. Click **Restart** to relaunch the app with package identity (the status should now show the package full name).
+7. Click **Launch Secondary App** to launch the secondary app entry point with package identity.
+
+To clean up, click **Unregister Package** and restart.
 
 ## Related links
 
 - [Windows App SDK](https://docs.microsoft.com/windows/apps/windows-app-sdk/)
 - [Single-project MSIX packaging](https://docs.microsoft.com/windows/apps/windows-app-sdk/single-project-msix)
-- [Sparse packages](https://docs.microsoft.com/windows/apps/desktop/modernize/grant-identity-to-nonpackaged-apps)
-- [WindowsAppSDK issue #5586](https://github.com/microsoft/WindowsAppSDK/issues/5586)
+- [Grant package identity by packaging with external location](https://docs.microsoft.com/windows/apps/desktop/modernize/grant-identity-to-nonpackaged-apps)
+- [IApplicationActivationManager::ActivateApplication](https://learn.microsoft.com/windows/win32/api/shobjidl_core/nf-shobjidl_core-iapplicationactivationmanager-activateapplication)