Search API Facets: https://www.elastic.co/guide/en/app-search/current/facets.html
What we have built for task type / topic selector is basically like faceted search, so it might make sense to use the ElasticSearch API (assuming it is available to us) for this purpose. One specific benefit is being able to get the count available for each facet, so you can show the user how many results are matching a specific task type / topic before they click on it.