JaLaBlog

I do not know the source of the following info:

“With the version of the Windows installer that comes with Win7, there is a new file at /Sources/ei.cfg that tells the Windows installer what OS it should install. Here’s what the file looks like from the ultimate disc: [EditionID] Ultimate [Channel] Retail [VL] 0
When the Windows installer sees this file, it becomes keyed to whatever distribution the file specifies. In this case with an Ultimate disc, we cannot install Home Premium or Professional. We have not yet had a chance to test OEM and retail keys since we don’t have both, however it certainly looks like the installer is going to make a distinction there too.
Perhaps the most interesting bit of this is that the installation process itself hasn’t changed – the Windows installer still supports One Disc to Rule Them All operation, and the Windows image that comes with Ultimate for example has absolutely no problem installing lesser versions. In fact if you remove ei.cfg so that the Windows installer is not told to install a specific version, it will revert to One Disc mode. The distinction between discs is entirely trivial, dictated by a single 51 byte configuration file. You can have a One Disc installer, Microsoft just don’t want it to be the default action for some reason.”