On AO3, users can put in whatever tags they want. (Autocomplete is there to help, but they don’t have to use it.) Then behind the scenes, human volunteers look up any new tags that no one else has used before and match them with any applicable existing tags, a process known as tag wrangling.
AO3’s trick is that it involves humans by design–around 350 volunteer tag wranglers in 2019, up from 160 people in 2012–who each spend a few hours a week deciding whether new tags should be treated as synonyms or subsets of existing tags, or simply left alone. AO3’s Tag Wrangling Chairs estimate that the group is on track to wrangle over 2 million never-before-used tags in 2019, up from around 1.5 million in 2018.