An APPCONTENT-MS file contains information about the content in a Windows app that allows enhanced in-app search capabilities. It stores the application content in XML format, which includes keywords and properties that are referenced when the app uses the Windows.Storage.Search API to perform the search.

You will most likely never encounter an APPCONTENT-MS file since it is stored deep in %Appdata%\Local directory. The files are not meant to be manually opened, instead, they are referenced by the app. However, you can manually open the file using a plain text or XML editor to view its contents since the file stores information in XML format. APPCONTENT-MS files may be used in a variety of applications, such as Facebook and Skype. The file may store various content depending on the app, such as users' full names, birthdates, phone numbers, home addresses, or the last time they signed in.