What are "Debug/Checked" versions of a Windows OS?

While looking for a Windows XP download in the MSDN subscriber portal, I noticed this:

So, what is the Debug/Checked version?

Solution 1:

The debug/checked version is used for debugging device drivers. It contains run-time checks for error conditions and has run-time optimizations removed.

Solution 2:

In particular, interpreting the explanations, using a Debug/Checked version is only likely to help you if you are writing kernel-mode components (most likely drivers), or if you are relying on some third-party kernel-mode components that you believe might be buggy under the conditions that you place them in.

That is to say: the vast majority of developers (i.e. those developing application software — software that users interact with, regardless of whether it's "managed" or "unmanaged", command-line or GUI-based; in short, user-space code) don't need this at all, and shouldn't use it, because it is slower than mainstream versions of Windows.

So if you're not sure whether you need it, you almost certainly should avoid it. Conversely, if your work would benefit from it, then you would already be well aware of it (e.g. from wherever it was that you learnt how to go about kernel-mode development).