This page is all about sending a private message to another user or group.
It is analogous to the "send to user" function of the Message Inbox, or to typing
/msg username ...
in the Chatterbox, but without all the other baggage those nodes carry.
The same underlying function sends the message in each of these cases, so you will see similar behavior in exceptional conditions, such as when the specified recipient does not exist, or if you attempt to send the same message twice.
Learn about how to link to the Message Outbox.