Unlike the generic Sub Form element found in the community edition, this AJAX Sub Form element gives you the flexibility of dynamically populating the sub form as the value it watches changes.
Example below shows that the AJAX Sub Form would change its content depending on the field's value it's tied to.
Figure 1: Configuration of AJAX Sub Form element.
Figure 2: AJAX Sub Form element is tied to the Multi-Select Form Element.
Figure 3: AJAX Sub Form changes it's content as when the field's value is changed.