Each Amazon Account is a single login to Amazon Seller Central, it represents one Amazon Merchant.

Each Amazon Merchant can sell on one or more Amazon marketplace using their Amazon account.

For example if you have a single North American Amazon account you could sell on one marketplace such as Amazon.com, or you could sell on 2 marketplaces Amazon.com and Amazon.ca. 

Likewise you may have a single Amazon Europe account and sell on only Amazon.de or only Amazon.co.uk. You could also have an Amazon Europe account and sell on Amazon.co.uk, Amazon.de and Amazon.fr (and even Amazon.it and Amazon.es too!).

In A2X - each A2X account connects to a single Amazon account and can access one or more of the marketplaces that account sells on depending on which subscription plan you select.