The total number of entries in the sitemap. The table may contain entries for sitemaps that are not fully built (is_active or complete_date will be 0).
Okay, kinda what I figured. I would assume it contains entries that can be crawled via permissions? Like I have 1078 threads on Snogs site, but the table shows 821. On OzzModz I have 12,202 threads and 13,671 entries. More than the thread count.
There is more to the sitemap than threads. There are at least addons that add content there (Marketplace / Images / ...).
I do think forums / subforums are in the sitemap too.