Significant terms aggregation is an evolution of the previous one, in that it's able to cover several scenarios, such as the following:
- Suggesting relevant terms related to current query text
- Discovering relations of terms
- Discovering common patterns in text
In these scenario, the result must not be as simple as the previous terms aggregations; it must be computed as a variance between a foreground set (generally the query) and a background one (a large bulk of data).