Simply put, the potential difference of any voltage source can also be termed as emf as it causes charge to flow. The potential difference across any other passive components cannot be termed as emf as it is not the cause of the current flow.

In physics, electromotive force, or most commonly emf (seldom capitalized), or (occasionally) electromotance is "that which tends to cause current (actual electrons and ions) to flow."More formally, emf is the external work expended per unit of charge to produce an electric potential difference across two open-circuited terminals....The created electrical potential difference drives current flow if a circuit is attached to the source of emf. ...Devices that can provide emf include voltaic cells, ......"A source of emf can be thought of as a kind of charge pump that acts to move positive charge from a point of low potential through its interior to a point of high potential

an EMF introduces energy into a circuit.

a PD dissipates energy in a circuit.