[arpinum]

The letters are different per-account.

[dkarp]

thanks, first time seeing that

[surfer7837]

It's so the load is evenly distributed across AZs because I bet most people just select -az-1.

[tedk-42]

As another commenters mentioned, the AZ letter different per account.

[asymptotic]

https://docs.aws.amazon.com/ram/latest/userguide/working-wit...

“To identify the location of your resources relative to your accounts, you must use the AZ ID, which is a unique and consistent identifier for an Availability Zone. For example, use1-az1 is an AZ ID for the us-east-1 Region and it is the same location in every AWS account”

So each separate AWS account will have a different AZ name that maps to use1-az1, but use1-az1 is a region-wide constant.