Tell me the difference between Call to existing action & Call to copy action in QTP?

In call to existing object, only the reference of the original object will be copied into the called action. And that too it will not be editable. When we need to use an existing objects with out modifying them, we can use call to existing action.

Where as call to copy action means calling the original action by itself. That means not an instance but the original object itself copied into the calling action. And of course it is editable. You can edit and change the action as desired.