Tags are a further way to select and organize sources.
By clicking a reference's tag, you can select / display all the sources in your library that also have that tag.
Note: references only display their first three tags; the reference's additional tags are noted by the tag with the + symbol.
To add a tag to a reference:
To search for sources with a certain tag:
Or click the tag of interest in a source to display all the sources with that tag.