Amazon uses zones that look like this: us-east-1d. I can find plenty of documentation about us-east-1, etc... but I do not see any reference to the a,b,c,d,e. What does this letter represent?
You can slipt it up like this, for example, consider us-east-1a:
Consider US West, there are two regions:
us-west-1 (North Virginia)
And both the region’s will have their own AZ’s IDs like a,b, or c.
Check out the Regions and Availability Zones documentation for more information.