I can't say I understand exactly what "Custom XML" is used for in Word 2007 and 2010. However, Microsoft lost the patent lawsuit over it to i4i.
ZDNet's Mary Jo Foley (back in August) seemed to be puzzled by it too. So, I'm not alone in trying to understand what the removal of this feature means.
Matt Asay commented 9also back in August) that it may affect working with the ODF format.
Here's Microsoft's MSDN overview of XML in Word in general with comments throughout about "custom XML".
Microsoft Office 2007 Service Pack 2 was released today and it is gigantic. Although Windows Update reported that it was 397.3MB large, the actual download was 509MB.
It is supposed to be faster (Outlook 2007 SP2 is said to be 26% faster) and more stable. It also provides built-in support for working with ODF and PDF files which previously required add-ons to do so.
Here's what I'd like to know though: Why does an Office update require Vista to reboot? That just seems wrong.
Vista's Windows Update didn't see any updates when I fired it up this morning. So, I clicked on the option to manually check for updates. It saw the Patch Tuesday Vista patches after that action. Why didn't it seem them as soon as it fired up? Then, of course, Vista wanted to be rebooted to actually finish installing the patches. Ugh. Vista takes a look time to shutdown and then reboot. Good thing I have multiple computers to work on.
After the reboot Word 2007 decided it wasn't to open a small DOCX file I had been working on. Now what? After futzing a bit, I checked the net and found that there is an Office 2007 SP1 update. I ran Windows Update again and, again, had it manually check for updates. Now, it sees the Office 2007 SP1 update. Why wasn't this detected earlier. To make matters worse, the Office 2007 SP1 update was 251MB large and took a while to download. And, of course, it too required a reboot after installing itself. Why in the world with an application need a reboot to install itself. The only two things I can think of that requires a Linux box to reboot are kernel or driver updates.
This is a broken and time-wasting scenario.