On a WordPress multisite network each blog has its own categories, tags and custom taxonomies. Keeping taxonomies separate is the default setup, but it is possible to have all networked blogs share the same terms.
The new Multisite Global Terms plugin allows sites to share global categories, tags and custom taxonomies across a multisite network. Created by Brajesh Singh of BuddyDev, this plugin makes it possible for the main site to share its terms_taxonomy table with the rest of the sites on the network.
If a new taxonomy term is added on any site, the Multisite Global Terms plugin makes it available to the rest of the networked sites. There are no visible changes in the admin except that each site will be able to see all available terms. It even works for custom taxonomies as long as the taxonomy is registered on the site where you’re trying to use it.
One point of caution: When you activate this plugin to allow global terms across all blogs, any existing terms on sub-sites that aren’t already available on the main site, will not be automatically incorporated. They will be lost. This is important to understand before activating, as it makes a major change. Multisite Global Terms is probably better to use if you’re setting up your multisite network to share global terms from the outset, as opposed to implementing it after many terms have already been created on sub-sites.
Sharing taxonomies across a multisite network can be useful if the sites are dedicated to a similar topic, ie. a network of real estate bloggers or any kind of niche blogging network. Content within a shared category is then better connected to related posts on the network. It’s also easier to theme a site to display recent posts from any blog across the network within a specific category, tag or custom taxonomy. The Multisite Global Terms plugin is available on WordPress.org. It must be network-activated after installation.