You can move all of a training object's text objects to the root category. This can be useful if you are dissatisfied with the classification of the text objects and want to start assigning them to categories over again from scratch.
Before you do this it is advisable to make a backup copy of the training object.
Then proceed as follows:
On the left hand pane, select the root Training Objects node to give the two-pane view.
On the right-hand pane, right-click a training object.
Choose Copy All Mails to Root Category.