Used to copy a value from a source field of the origin issue to a destination field of a new issue.
You can add any number of Copy to New configurations.
Be aware, that all destination fields must also be on the create issue screen of the new issue.
Tip: You might set up a special transition screen to mainly copy those fields to the newly created issue. So it feels more like it would be the create screen of the new issue. In that case, you won't show those fields on the origin issue, but reset them again afterward (just empty or back to the default value), see Copy within the Origin Issue.
For 'Copy from the Origin Issue to the New Issue' append & prepend only make sense if the target field contains already a default value. So in practice, append or prepend are more often used for 'Copy within the Origin Issue', if at all.