Variable number of colons in AWS ARNs

Question

I'm looking up docs on S3 and I noticed the full ARN contains three colons (:) at the end, IAMs have 2, etc. Is there a reason for it or any logic?

Answer 1

Look at the top of the page you linked, where it explains the ARN format:

arn:partition:service:region:account-id:resource-id

The IAM ARNs in the examples at the bottom of the page leave the region value blank.

The S3 ARNs in the examples at the bottom of the page leave both the region and account-id values blank.