Using a Plist
Apps built for iOS and Mac consist of an “application bundle” which contains the app itself, resources and other components such as frameworks. It also contains an “Info.plist” file, which is a special XML file in Apple's "property list" format containing specific settings that tell the OS about your app.
For certain apps, you may need to modify the plist to enable features. To make it easier for you to include your own settings in the app plist file, you can create your own Info.plist file with the specific settings you need and drag it into your Xojo project. When your app is built, the settings in your plist file are copied to the app plist file.
One example of when you might need a plist to to deal with Using Non-Secure URLs on macOS and iOS on iOS or macOS 10.11 or later when connecting to http URLs.
General Plist guidelines
Do not add more than one plist file to a project.
Any items in your plist file that are duplicates of what is created during the build process are overwritten by the build process.
Only top-level keys and their entire value are copied. For example, if a key specifies a dict for a value then the entire dict is copied. Top-level keys are keys that are immediate children of PLIST > DICT in the plist XML structure.
The file must have both a plist header and the extension ".plist" before you add them to a project.
Refer to Apple's docs for more details about plist and keys:
URLConnection, DesktopHTMLViewer, MobileHTMLViewer classes; Using Non-Secure URLs on macOS and iOS topic