In this dialog, you can perform two operations:
Change order of accounts
You may change order how accounts are shown in main Account list.
ⓘ First enabled account is the default account.
Accounts that you don’t wish to see in Account list, but still want to keep them around (not to delete them) may be disabled. Such disabled accounts won’t be shown, won’t be checked for mail, and won’t be invisible in all menus where accounts are shown.