A driver is an object with many outputs, a startup program, for example. A utility is an object with many inputs, DATEFMT, for example. Filtering drivers and utilities from large diagrams reduces diagram clutter and typically provides a better picture of what your application does.
You set thresholds for filtering in the Diagrams User Preferences. If you set the driver threshold for outgoing relationships to 40, for example, then any object with 40 or more outgoing relationship will be considered a driver and displayed outside the main relationship flow. The driver and its related nodes in the relationship flow appear with a symbol in the diagram. Click the symbol to display the relationships for the driver and its related nodes again.