Here are the various fields of the dialog.
You can turn the mechanism on an off without changing the vacation text. Turning off the mechanism erase the correspondent list.
The vacation notice is sent only once to the various correspondent. A database is maintained to remember who has already received the notice. You may want to reset this database so the notice is sent again. In general you do this because you have changed the vacation text (your holiday will last longer than expected for example).
This will be used as the subject. Strings like "out of town" or "unavailable" may be used.
This will form the body of the message sent. In text mode, to enter edit mode in a text area widget, just hit "SPACE". Enter the text and hit "ESC" to return to normal navigation mode (allowing you to use "TAB" to reach the buttons).