A useful feature of Rollup is the ability to query both "live", realtime data in addition to historical "rolled" data in a single query.
For example, your system may keep a month of raw data. After a month, it is rolled up into historical summaries using Rollup and the raw data is deleted.
If you were to query the raw data, you’d only see the most recent month. And if you were to query the rolled up data, you would only see data older than a month. The RollupSearch endpoint, however, supports querying both at the same time. It will take the results from both data sources and merge them together. If there is overlap between the "live" and "rolled" data, live data is preferred to increase accuracy.