Options for the user filter are created with the a!facetOption() function. The most popular use of a!facet() is to generate filter options instead of manually writing them. See the Expressible User Filters page for common uses.
There are older versions of this function. You can identify older versions by looking at the name to see if there is a version suffix. If you are using an old version, be sure to refer to the corresponding documentation from the list below.