Kaffārah (expiation) is prescribed as a penalty for invalidating a Ramadan fast after it has been commenced. If the fast was not begun in the first place, or if a fast outside of Ramadan is invalidated, then only qaḍāʾ (making up the fast) is required. In this respect, there is no distinction between whether the fast is obligatory (farḍ), necessary (wājib), or voluntary (nafl).
However, an important reminder should be noted. The Prophet (peace and blessings be upon him) stated in a ḥadīth that whoever fails to fast one day of Ramadan—without a valid excuse—would not attain the reward of that single day even if he were to fast for the entirety of the year in its place.
Moreover, for a Muslim to abandon the Ramadan fast without a legitimate excuse constitutes a grave sin. In such a case, beyond making up the missed day through qaḍāʾ, the individual must also turn to Allah, Exalted and Glorious, in sincere repentance (tawbah) and seek His forgiveness (istighfār) for the sin committed.