A user tag is like a little note you attach to a user’s profile.
If you’re at all familiar with RES for Reddit, it works the same way ~~except I don’t believe it keeps a history of why the tag was added as is the case with RES.~~
To enable it go to your Settings > User Tags > Enable User Tags. From there you can also track how many up and downvotes you’ve given someone. One item of note, however, is that the tags don’t sync across devices, so if you use Voyager on both iPhone and iPad the recorded information won’t match.
