With the filter parameter, after a user selects a facet value for filtering, the other facet values in that field are not returned by the API and are no longer visible.
The filter_facet parameter allows other facet values to be returned after a facet value has been selected.
filter_facet doesn't support complex boolean operations, but they can be supported by including both filter and filter_facet.
Note that the attributes used in filter will not return all values after the user selects a facet value in that field.
Example values:
Single filter with multiple values
&filter_facet=color:"red" OR "purple"
Multiple filters on different attributes
&filter_facet=color:"red"&filter_facet=brand:"Bloomreach"