The filters list the ten most popular tags for each tag category. To filter by any other tags, use the "Other tags to include" field.
If the tag that interests you isn't in the top ten, start entering the tag you need in the "Other tags to include" field—all tag categories are valid here, and you can add as many tags as you want. The autocomplete will help you find the canonical version of the tag. Use these to take full advantage of the wrangled tagging structure, which will find all works using subtags and tags with synonymous meanings.
You can also enter tags that aren't in the autocomplete. If the tag you enter has been used on the Archive but is not marked canonical, then the filters will look for works that use the exact tag you've entered. If the tag you've entered has never been used on the Archive, the filters will do a simple text match and may bring up unexpected results. The "Search within results" field will text-match more accurately, especially in the case of relationship tags and other tags with "/" or other non-text characters.
Choosing any tag from a category or entering a tag in the "Other tags to include" field will do an AND search with all the tags you select. This means that if you're filtering the works tagged with the F/F category, select the Teen and Up Audiences rating, the canonical Romance tag in the Additional Tags category, and enter or select the canonical tag Drama in the "Other tags to include" field, only works tagged with all these tags will be included in the results.
To get results with Tag A OR Tag B, use the "Search within results" field.
To look up tags and see which ones are canonical, use the Tag Search.
You can find more information about tags in our Tags FAQ. To read the guidelines tag wranglers use to mark tags canonical, among other things, or to better understand the AO3-specific vocabulary for tags, read the Wrangling Guidelines.