Once upon a time, Amazon offered four tiers of object storage:
Standard S3 (this is S3 as commonly discussed) Infrequent Access S3 (costs less, but you pay to access it more frequently) Reduced Redundancy S3 (similar to standard S3, but offers 4 9's of durability instead of Standard S3's 11 9's) Glacier (long term storage that doesn't need to be available instantly) Officially, Reduced Redundancy S3 still exists. That said, it's apparent that Amazon isn't moving forward with the offering, based upon two datapoints:
Second, and more relevant to optimizing costs, S3-RR no longer participates in service discounts that affect the other S3 tiers.
As a direct result, while S3 Reduced Redundancy still exists, you will pay more to store objects there than you will in Standard S3, while achieving worse object durability.
In conclusion, if you've got any objects living in Reduced Redundancy, it's time to migrate them to standard S3. Your bill and your data durability will both thank you for it.