Understand load behaviour of modification packages

Thanks for your question! Here is a description of what happens when you have a main package and a modification packages. 

Scenario 1: Main package with help.txt in vfs with no modification packages

When the app runs, main app will load help.txt.

Scenario 2: Main package with help.txt in vfs with modification packages that contains no help.txt in vfs

When the app runs, the main app will load help.txt from the main app

Scenario 3: Main package with help.txt in vfs with modification package that contains help.txt in vfs

When the app runs, the main package will load help.txt from the main app. This is because we found that the main package will always win over the modification packages.

Scenario 4: main package does not contain help.txt in vfs with modification package that contains help.txt in vfs. This is because when we do a look up, help.txt is first found in the modification package.

When the app runs, the main app will load help.txt from the modification package

Scenario 5: main package does not contain help.txt in vfs with modification package #1 (amodi.msix) and modification package #2 (bmodi.msix), both having help.txt in vfs.

When the app runs, the main app will load help.txt from modification package #1. This is because when we do a look up, we found help.txt first in amodi.msix.

In the next update, we will be fixing scenario #3, so that modification packages can overlay their files correctly when loading the main package. More details will be posted at http://aka.ms/msix soon. 

More info on modification packages in the current release visit https://docs.microsoft.com/en-us/windows/msix/modification-package-1809-update