Longer EC2 Resource IDs Now Available
aws.amazon.com/blogs/aws/theyre-here-longer-ec2-resource-ids-now-available
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Effective today, you can opt in to the new format for EC2 instances and EC2 reservations. Each reservation represents the results of a single instance launch request, and can be associated with multiple instances.