The help text of aspnet_regiis utility says the following for "-ir" option:
Install this version of ASP.NET, register only. Do not change any web applications to use this version.
And the following for "-iru" option:
Install this version of ASP.NET. If there are any existing applications that uses ASP.NET, it will not change IIS configuration to use this version.
It must be my English, but I do not understand what the difference is - both won't change any apps to use this version. And I'm afraid that trying this out myself won't be precise, since I hardly know where to look into...
Does anyone know the difference?