The Hold command is used to put both legs of the call into Hold state. You can only change back the callstates to InCall state with Unhold command.
(Find out what kind of CallStates a call can have.)
If a decision needs to be made about the call, it's advised to put both parties to Hold state with this command.
And after the decision has been made you can set the call back to InCall state, transfer the parties or hang it up.
<Speak Party="all">This call will be put on hold</Speak>