Threads are processed differently depending on IIS5. or IIS 6 is used. For each request that comes in, a new instance of the appropriate HttpApplication-derived class is created, as are the associated modules for that app. To avoid reallocating apps and modules too often, each AppDomain maintains a pool of apps and modules. The maximum size of the app pool is the same as the size of the thread pool, so by default, up to 25 requests per worker process can be processed concurrently, each with its own app and module set.