Content libraries are container objects for VM and vApp templates and other types of files, such as ISO images. A standard content library is hosted by a vCenter Server. For normal production use, this works just fine as your vCenter server is always kept running. Many customers and partners run lab or demo environments that are frequently destroyed and recreated. In these situations, admins often turn to S3 to host a content library that persists across environment teardowns. Although this is not an officially supported by VMware, it is a widely used technique in the community.
This AWS re:Post Article demonstrates how to use the the manual method for indexing S3 Content Libraries first, then goes on to demonstrate the automated method.