The is a behind-the-scenes feature within IIS Express (a lightweight, self-contained version of IIS for developers). Its primary function is to emulate the behavior of different versions of full IIS (Internet Information Services) to ensure that legacy web applications run correctly without code changes. It acts as a shim layer, intercepting API calls and feature requests from an application and translating them to match the expected behavior of an older or different IIS version.
For critical legacy apps, test on a real IIS VM in addition to using IIS Express compatibility mode. iis express application compatibility database
When you run IIS Express, it checks the Application Compatibility Database to determine if there are any known compatibility issues with the applications or technologies you're using. If a match is found, IIS Express may apply specific configuration settings or display warnings to help you avoid potential issues. The is a behind-the-scenes feature within IIS Express