This article explains the function of shared tags.
When multiple tagged files are selected, Shared tags are displayed in the batch processing panel. These are tags that are assigned to all selected files.
Below the shared tag section, Additional tags are listed. These tags are assigned to at least one file. The number shown in parentheses indicates the exact number of files the tag is assigned to. Tags from this section can be added to all selected files by clicking them. They will then appear under Shared tags.