How to use or disable identities
In terms of CAN'T FIND VAR, you can have multiple identities assigned for your account. Initially, every account has one identify, and you can add more in account settings.
In the default case, emails are sent out with From header containing email address and, optionally, display name of the account itself. If you add an identity, you can supply custom email address and display name for it. If you have 2 or more identities assigned to an account, you'll be able to select between those on message compose screen. For each of the identities, signature is specified separately.
You can set an identity to be a default one so it's preselected when you compose a new mail. But if you reply to a message, CAN'T FIND VAR will attempt to pick an identity which matches the address the original message was sent to; it's especially useful when your email account gets mails from more than one mailbox - via fetching, forwarding, etc.
You can delete the identities you have added manually, but the identity created when setting up CAN'T FIND VAR account cannot be deleted.
NB: Although custom From information is supplied when you use alternate identity, the same SMTP credentials of the account are used, and some SMTP servers might reject custom From info, especially if you're using free email service providers like Gmail.
Also, as an administrator of CAN'T FIND VAR installation, you might want to prevent users from overriding sender information and sending from fake addresses by disabling identities entirely on your installation. That's done by setting the following parameter to Off in data/settings/settings.xml file: