Multisite global terms plugin allows you to share your categories/tags or any other taxonomy terms across the whole WordPress Multisite installation. It uses the main site tables to store all terms. The terms can be added from any of the blog and will be accessible across all the blogs on the multisite installation.
Event the custom taxonomy terms will be available across the sites if the taxonomy is registered on multiple blogs. If say a custom taxonomy is registered on only two of the blogs, the terms in those custom taxonomies will be available only on those two blogs.
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