Here’s Amazon’s description of RRS:
“Amazon S3’s standard and reduced redundancy options both store data in multiple facilities and on multiple devices, but with RRS, data is replicated fewer times, so the cost is less. Amazon S3 standard storage is designed to provide 99.999999999% durability and to sustain the concurrent loss of data in two facilities, while RRS is designed to provide 99.99% durability and to sustain the loss of data in a single facility.”
and of Standard-IA:
Amazon S3 Standard – Infrequent Access (Standard – IA) is an Amazon S3 storage class for data that is accessed less frequently, but requires rapid access when needed. Standard – IA offers the high durability, throughput, and low latency of Amazon S3 Standard, with a low per GB storage price and per GB retrieval fee. This combination of low cost and high performance make Standard – IA ideal for long-term storage, backups, and as a data store for disaster recovery. The Standard – IA storage class is set at the object level and can exist in the same bucket as Standard, allowing you to use lifecycle policies to automatically transition objects between storage classes without any application changes.