In-process mode simply means using ASP.NET session state in a similar manner to classic ASP session state. That is, session state is managed in process and if the process is re-cycled, state is lost. Given the new settings that ASP.NET provides, you might wonder why you would ever use this mode. The reasoning is quite simple: performance. The performance of session state, e.g. the time it takes to read from and write to the session state dictionary, will be much faster when the memory read to and from is in process, as cross-process calls add overhead when data is marshaled back and forth or possibly read from SQL Server.
In-process mode is the default setting for ASP.NET. When this setting is used, the only other session config.web settings used are cookieless and timeout.
If we call SessionState.aspx, set a session state value, and stop and start the ASP.NET process (iisreset), the value set before the process was cycled will be lost.