This article explains how files can be tagged using batch processing.
First, select any number of files, then choose Tags from the action bar at the bottom. The batch processing panel opens. Under Tag list, you can search for and select existing tags or create new ones. Confirm by clicking Save.
Shared tags are displayed at the top. Tags that are present in some, but not all, selected files are shown below. Clicking these tags adds them to all selected files.