This is what it means...
"Use of this OEM System Builder Channel software is subject to the terms of the Microsoft OEM System Builder License. This software is intended for pre-installation on a new personal computer for resale. This OEM System Builder Channel software requires the assembler to provide end user support for the Windows software and cannot be transferred to another computer once it is installed. To acquire Windows software with support provided by Microsoft please see our full package "Retail" product offerings."
If you buy that, it's expected you're reselling the computer you're building. As such, you'd be the support for whoever buys it. Microsoft doesn't provide support for the OEM versions if anyone has problems.
The person using the computer gets all the things through Windows Update like usual though.
The OEM version also normally comes with an activation scheme that's a bit different, and it gets tied to the individual machine. You can't transfer over to something else later if you'd need to, unlike normal activation.
I'd also add that unless you're sure you need a couple features like BitLocker and the ability to use language packs this round, there's likely no need for the Ultimate version. The extra features it includes -- identical to Enterprise -- are mainly useful in business environments. Ultimate is really just the way for consumers to get that version this round, though most people don't need it. There aren't Ultimate "Extras" unlike happened (sort of) with Vista, and all versions will perform the same.
Hope this helps