I think that used to be the sitewide category option which is gone.
Per things you need to know
Categories and Tags
Currently, there are global tags in WordPress 3.0, but no global categories. You can use the Sitewide Tags WordPress Plugin or other similar Plugins to incorporate global tags on the portal/front page of the site or on specific pages or blogs within the network to increase navigation based upon micro-categorized content.
http://ww.wp.xz.cn/extend/plugins/wordpress-mu-sitewide-tags/
It was just a default setting within WPMU (no plugins were required).
There’s mention of global_terms_enabled (available from Super Admin > Options), but I can’t see that option on any of my installs (some upgrades, some new installs).
I believe that if I can activate this, this might sort it.
Any ideas?
The checkbox to enable it was removed in post 3.x.
Dang, I really need some global categories.
I’ve searched high and low for solutions but all the threads I find have this super complicated mix of Sitewide Tag Pages, New Blog Defaults, and one or two other plugins, and I can’t even get New Blog Defaults to work! (it won’t replace the Uncategorized cat in new blogs)
The original global categories didn’t work the way you need it to work. It just aggregated all categories to keep track of them. It didn’t push put set categories for each blog.