Category Bulk Tagging

Bulk Category Tagging lives under your "Bulk Edit Products Within Category" option. Select this to get started with bulk tagging. 

Then scroll down and select: 

Then you can select your tags to apply to your categories. Tags you want applied to all products within the category you have selected will appear under "Shared Tags". "Partially Shared Tags" are those tags that have been applied to some products within this category, but not all.