For IIS Application Pools there are two options – Integrated Pipeline mode and Classic Pipeline mode.

To handle ASP.NET applications, IIS versions before 7 integrated ASP.NET with IIS via an ISAPI extension, and exposed  its own application and request processing model. Thus there were two separate server pipelines, one for handling native ISAPI filters and another pipeline for managed application components (like ASP.NET).

With IIS 7, the ASP.NET runtime  is integrated into the core web server. And so new ASP.NET apps should use the Integrated pipeline. Depending on their configuration legacy ASP.NET apps may need to be run using the Classic pipeline.