The top hit aggregation is different from the other aggregation types: all the previous aggregations have metric (simple values) or bucket values; the top hit aggregation returns buckets of search hits (documents).
Generally, the top hit aggregation is used as a sub-aggregation, so that the top matching documents can be aggregated in buckets. The most common scenario for this aggregation is to have, for example, the top n documents grouped by category (very common in search results in e-commerce websites).
