Saved account name in main login screen if "Remember Account Name" option is enabled, and using a Battle.net account, in which case the CVar_accountName should be set to the correct email address.
The CVar is a list of account names delimited by pipe characters (|). The last account used is marked with an exclamation point (!) before the name of the account. All accounts regardless of zone (US/EU etc.) are listed. The accounts are listed (when they come from the server) in the order that they were linked with the Battle.net account.
More info: (about the phrasing of the CVar's text)
When the client saves the CVar (or maybe when the client gets the CVar from the server) there is a trailing pipe character. When the client phrases the CVar, it excludes blank entries. The last server with an exclamation point becomes the default server. If there are no entires with an exclamation point the combo box still shows up but no entries are selected. If there are multiple entries with the same name, they will all be displayed in wow and they all can be selected at the same time. The order the account names are displayed in is the order that they appears in the CVar. This CVar is difficult to view in the console because the console will attempt to render the pipe characters as escape sequences similar to how chat frames render colored text and item links.
SET accountList "!account1|account2|" SET accountList "account1|!account2|"
SET accountList ""