It's here, right on the 1st post!
All linked to wayback machine in this post.
No, WSUS Offline doesn't download updates for WEPOS, WES09 and POSReady 2009, they don't contain XP in their names. Older WSUS Offline versions downloaded updates for XP Embedded, but these updates were filtered out starting from 9.2.2 build. However, you can still download version 9.2.1 and use it.
WSUS Offline downloads current WSUS database, but it also create its own database (which may provide links for third party tools, custom, non-public updates or whatever the developer finds appr