Now i would like to know why you make a difference between sunna and nafl?
Because for me it is the same, maybe only under some circumstances in (technical term) fiqh language اصطلاح الفقهاء it makes a slight difference of the level of tawab. But for example all nafl is sunna!
And in general Nafl=Sunna (they are synonyms) as it is a kind of "extra worship" the Messenger of Allah (peace be upon him) made which is not prescribed by Allah in the Quran...
This meaning can be taken out of the following Hadith:
On the authority of Abu Hurayrah (may Allah be pleased with him) who said: The Messenger of Allah (peace and blessings of Allah be upon him) said, “Verily Allah ta’ala has said: ‘Whosoever shows enmity to a wali (friend) of Mine, then I have declared war against him. And My servant does not draw near to Me with anything more loved to Me than the religious duties I have obligated upon him. And My servant continues to draw near to me with nafil (supererogatory) deeds until I Love him. When I Love him, I am his hearing with which he hears, and his sight with which he sees, and his hand with which he strikes, and his foot with which he walks. Were he to ask [something] of Me, I would surely give it to him; and were he to seek refuge with Me, I would surely grant him refuge.’ ” [Al-Bukhari]
عَنْ أَبِي هُرَيْرَة رَضِيَ اللهُ عَنْهُ قَالَ: قَالَ رَسُول اللَّهِ صلى الله عليه و سلم إنَّ اللَّهَ تَعَالَى قَالَ: "مَنْ عَادَى لِي وَلِيًّا فَقْد آذَنْتهُ بِالْحَرْبِ، وَمَا تَقَرَّبَ إلَيَّ عَبْدِي بِشَيْءٍ أَحَبَّ إلَيَّ مِمَّا افْتَرَضْتُهُ عَلَيْهِ، وَلَا يَزَالُ عَبْدِي يَتَقَرَّبُ إلَيَّ بِالنَّوَافِلِ حَتَّى أُحِبَّهُ، فَإِذَا أَحْبَبْتُهُ كُنْت سَمْعَهُ الَّذِي يَسْمَعُ بِهِ، وَبَصَرَهُ الَّذِي يُبْصِرُ بِهِ، وَيَدَهُ الَّتِي يَبْطِشُ بِهَا، وَرِجْلَهُ الَّتِي يَمْشِي بِهَا، وَلَئِنْ سَأَلَنِي لَأُعْطِيَنَّهُ، وَلَئِنْ اسْتَعَاذَنِي لَأُعِيذَنَّهُ".