In addition to creating lists and nested sublists to organize your library, you can add one or more searchable keyword Tags to any reference.To create a Tag, just click the + in the Tags field in the right-hand side panel, under the abstract:
Any keywords you create will be listed in the left menu panel, under the Tags section. All tags become links - clicking on a tag will take you to the list with all references containing the same tag.
Easily bulk tag articles from your library. Select the references you want to bulk tag, then drag and drop onto the Tag from the list. The references will now all have the associated tag.
It's also quick and easy to search for any tag using the general Search bar at the top. You also can create more complex search queries using multiple tags using AND or OR. Read more about advanced search options here.
Tags act as their own annotation layer meaning you can tag a paper "XYX" and also add it to multiple lists. Your tag library from your personal library can be unique from the tags in any shared library.
On the Tag list next to each keyword you will find a settings gear which will allow you to edit or delete the tag. Changing anything on the tag settings will change for all articles associated with the tag.