+
+ <para>
+ Messages may have a <literal>DESTINATION</literal> field (see <xref
+ linkend="message-protocol-header-fields"/>), resulting in a
+ <firstterm>unicast message</firstterm>. If the
+ <literal>DESTINATION</literal> field is present, it specifies a message
+ recipient by name. Method calls and replies normally specify this field.
+ The message bus must send messages (of any type) with the
+ <literal>DESTINATION</literal> field set to the specified recipient,
+ regardless of whether the recipient has set up a match rule matching
+ the message.
+ </para>
+
+ <para>
+ When the message bus receives a signal, if the
+ <literal>DESTINATION</literal> field is absent, it is considered to
+ be a <firstterm>broadcast signal</firstterm>, and is sent to all
+ applications with <firstterm>message matching rules</firstterm> that
+ match the message. Most signal messages are broadcasts.
+ </para>
+
+ <para>
+ Unicast signal messages (those with a <literal>DESTINATION</literal>
+ field) are not commonly used, but they are treated like any unicast
+ message: they are delivered to the specified receipient,
+ regardless of its match rules. One use for unicast signals is to
+ avoid a race condition in which a signal is emitted before the intended
+ recipient can call <xref linkend="bus-messages-add-match"/> to
+ receive that signal: if the signal is sent directly to that recipient
+ using a unicast message, it does not need to add a match rule at all,
+ and there is no race condition. Another use for unicast signals,
+ on message buses whose security policy prevents eavesdropping, is to
+ send sensitive information which should only be visible to one
+ recipient.
+ </para>
+
+ <para>
+ When the message bus receives a method call, if the
+ <literal>DESTINATION</literal> field is absent, the call is taken to be
+ a standard one-to-one message and interpreted by the message bus
+ itself. For example, sending an
+ <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
+ <literal>DESTINATION</literal> will cause the message bus itself to
+ reply to the ping immediately; the message bus will not make this
+ message visible to other applications.
+ </para>
+
+ <para>
+ Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
+ the ping message were sent with a <literal>DESTINATION</literal> name of
+ <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
+ forwarded, and the Yoyodyne Corporation screensaver application would be
+ expected to reply to the ping.
+ </para>
+