Availability Zone and Region

Quite sometime i was trying to understand the concept of Availability Zone and Regions.

Finally got this answer from Amazon .

When you launch an instance, you can optionally specify an Availability Zone. If you do not specify an Availability Zone, Amazon EC2 selects one for you in the Region that you are using. When launching your initial instances, we recommend accepting the default Availability Zone, which allows Amazon EC2 to select the best Availability Zone for you based on system health and available capacity. Even if you have other instances running, you might consider not specifying an Availability Zone if your new instances do not need to be close to, or separated from, your existing instances.

[Note] Note
Availability Zones are not the same across accounts. The Availability Zone us-east-1a for account A is not necessarily the same as us-east-1a for account B. Zone assignments are mapped independently for each account.

You are charged a small bandwidth charge for data that crosses Availability Zones “

Can I use the same SSH key pair across Regions?

No. You must create a separate SSH key pair for each Region.

[Note] Note
This is the key pair used for SSH connections to the instance. Your AWS Account ID credentials are global and you use them for all Regions.