You're right, the email alarm is for an email, text, or app alert/notification.
The other 2 are only available if you have a Display module connected. Otherwise, they aren't even shown.
They activate alert sounds from the Display module when the alert conditions are met.
To choose which sounds for which, go to the Misc page, and set them under Sounds on the right.
The Display sounds are pretty anemic; they're unlikely to wake you in the next room, for example.
But, it's possible to hook up a much louder buzzer, and have it also activate when either Display sound alarm is on.