Express Application Compatibility Database [upd]: Iis

Because IIS Express runs as a normal process rather than a system service, the database helps manage permissions so developers can debug applications without elevated administrative privileges. Installation and Registry Presence

In applicationhost.config , under <system.applicationHost> or <sites> , you can specify: iis express application compatibility database

It handles things like UAC (User Account Control) virtualization, shim engines for older OS versions, and specific privilege escalation so your web server can bind to ports without you having to manually run everything as Administrator every single time. Because IIS Express runs as a normal process

The database serves several purposes: