_BackupSetInstancesNote

A Backup Set allows to retain an isolated use case oriented set of the last at most N images of one or more EC2 instances.

When the EC2 Image task's Grow & Prune Image Backup Set action executes, it will:

1. Create a new EC2 image for each selected EC2 instance and tag the image with a key Backup Policy and a value based on the Backup Set name (thus "grow the image backup set")
2. Enumerate all existing images for the selected EC2 instance(s) that are tagged with a key Backup Policy whose value matches the Backup Set name
3. If it finds more images in this Backup Set than defined by the Backup Retention number, it will delete the oldest images in FIFO order until only the desired number is left (thus "prune the image backup set")

While there are also separate Grow Image Backup Set and Prune Image Backup Set actions to separate the two activities (for example, to only grow the backup set on each work day, but prune it to the last work day's image at the end of the week), the Grow & Prune Image Backup Set action covers most rotating backup use cases in a single operation.