Comment by twoodfin

I interned with the AppCompat team shortly before the release of Windows XP, which was huge for them as it was the first Windows for consumers on the NT kernel.

IIRC, they had a significant lab and tons of infrastructure for exercising and identifying compatibility issues in thousands of popular and less popular software packages. It all got distilled into a huge database of app fingerprints and corresponding compatibility shims to be applied at runtime.