Sunnah mu’akkadah (confirmed Sunnah), naafil (supererogatory), voluntary and mandoob (recommended) all share a similar meaning; they are acts of worship that are enjoined and encouraged in Islam, without being obligatory. The one who does them will be rewarded but there is no sin on the one who does not do them.
That is like praying qiyaam al-layl (optional prayers at night), the sunan rawaatib (regular Sunnah prayers), starting on the right when putting on one’s clothes, and so on. Some of the scholars think that these words are similar in meaning, whilst others – like the Maalikis – differentiate between them. In their view “Sunnah” refers to something that the Prophet (blessings and peace of Allah be upon him) did persistently; naafil refers to things that he did sometimes and not at other times.
Ad-Dasooqi al-Maaliki said: Naafil refers in linguistic terms to something extra or additional; in Islamic terminology it refers to that which the Prophet (blessings and peace of Allah be upon him) did but did not do persistently, i.e., sometimes he did not do it and sometimes he did do it. It does not mean that he stopped doing it altogether, because one of his characteristics is that if he did a righteous deed he would not stop doing it altogether after that. … With regard to “Sunnah”, in linguistic terms it refers to a way or path; in Islamic terminology it refers to that which the Prophet (blessings and peace of Allah be upon him) did openly when he was among a group of people, and he persisted in doing it, but there is no proof to indicate that it is obligatory. Sunnahs that are described as mu’akkadah (confirmed) are those that bring a great deal of reward, such as Witr.
End quote from Haashiyat ad-Dasooqi, 1/312
Al-Khateeb ash-Sharbeeni ash-Shaafa‘i said: Chapter on offering naafil prayers. Naafil in linguistic terms refers to something extra; in Islamic terminology it refers to actions other than those which are obligatory. They are so called because they are extra to what Allah, may He be exalted, has made obligatory. Naafil is similar to Sunnah, mandoob (recommended), mustahabb (encouraged) and so on. This is the well-known view.
End quote from Mughni al-Muhtaaj, 1/449
Ibn an-Najjaar al-Hanbali said: That which is mandoob (recommended) is called Sunnah, mustahabb, naafil, and so on… The highest of that which is recommended is Sunnah, then fadeelah (virtue), then naafilah.
End quote from Sharh al-Kawkab al-Muneer, p. 126
It should be noted that the Hanafis regard the one who does not do Sunnah mu’akkadah (confirmed Sunnah) actions as having sinned, but they say that his sin is less serious than that of one who fails to do obligatory (waajib) actions.
There is no difference of opinion among the scholars concerning the fact that some Sunnah actions are more confirmed and bring greater reward than others. Thus it may seem that this difference of opinion is the matter of a difference in names only; with regard to the meaning, there is no difference of opinion concerning it.
Ibn Nujaym al-Hanafi said: What appears to be the case from the words of our fellow scholars in our madhhab is that sin is incurred by failing to do obligatory (waajib) or sunnah mu’akkadah actions, according to the correct view, because they clearly stated that the one who omits the sunnahs of the five daily prayers is not said to have sinned, but the correct view is that he is sinning. This was stated in Fath al-Qadeer. And they clearly stated that the one who fails to pray in congregation is sinning even though it is a sunnah mu’akkadah according to the correct view. And there are other similar examples to be seen by the one who studies their words. Undoubtedly the sin in some cases is greater than in others, and the sin of one who does not do the sunnah mu’akkadah action is less serious than that of the one who does not do the obligatory (waajib) action.
End quote from al-Bahr ar-Raa’iq, 1/319
And Allah knows best.