https://docs.splunk.com/Documentation/Splunk/8.1.1/Indexer/HowSplunkstoresindexes

Once further conditions are met (for example, the index reaches some maximum number of warm buckets), the indexer begins to roll the warm buckets to cold, based on their age. It always selects the oldest warm bucket to roll to cold. Buckets continue to roll to cold as they age in this manner. Cold buckets reside in a different location from hot and warm buckets. You can configure the location so that cold buckets reside on cheaper storage.