This document discusses how I/O threads work in the ASP.NET application model. The worker process uses I/O completion ports and a thread pool to handle asynchronous I/O operations from multiple application domains without blocking threads. When the thread pool is depleted, worker threads wait for pending I/O operations to complete before processing new requests. It also covers the normal and asynchronous page lifecycles in ASP.NET and shows how HTTP modules and handlers process requests.