The S3 multiple buckets sample is a new content store subsystem that's based on the StoreSelectorContentStore.
The Store selector has two stores (instances of the S3 content store):
store1.s3ContentStoreas the defaultstore2.s3ContentStoreas the second one
The sample files are found in alfresco-s3-connector-4.0.x.amp:
s3-multiple-buckets-context.xml.sampleinconfig/alfresco/extensions3-mb-contentstore-context.xml.sampleands3-mb-contentstore.properties.sampleare inconfig/alfresco/extension/subsystems/ContentStore/S3MultipleBuckets/S3MultipleBuckets