as salam alaikum
Some Sunni schools put emphasis on classifying the ritual prayers in Fard, Sunnah and Nafl. Accordingly, a Fard prayer is a mandatory prayer that Allah has prescribed on His servants (like the five daily prayers). A Sunnah prayer is complementary to the Fard prayer and it is the one that the believer should not miss without justification; however if there is a particular justification, it can be missed. A Nafl prayer is a optional prayer whose performance bring particular blessings but it is not compulsory in any case.
In the Shi'i world all supererogatory prayers are usually called "Nawafil" (plural of "Nafl"). There are ahadith in some Shi'i books, like "Tahdhib al-Ahkam" by Shaykh Tusi, from which we can derive the preference of some Nawafil more than others.
With prayers for your success.