From the help file:

Report layouts in ActiveReports for .NET 3.0 are automatically saved as C# or Visual Basic for .NET files within the confines of the project in which they are created. In this way, layout information models the behavior of Windows Forms in the .NET framework; however, report layouts can be saved as stand-alone RPX files—the XML-formatted file on which previous editions of ActiveReports for .NET were based. This functionality makes ActiveReports for .NET 3.0 truly backward compatible. Older layout files can easily be brought into the newest applications while new layout files can be saved to an older format.