Waalaykum-assalam, Thank you for writing to us. Muslims believe that God has only obliged them to offer two rak'āt in fajar, four in ẓuhr, four in aṣr, three in maghrib and four in ishā Prayer. There is no difference of opinion in the ummah that these are the obligations which people must fulfil. These are called maktūbāt.
What is sunnat-i mu'akkadah then? Sunnat-i Mu'akkadah is a term applied by the jurists to the optional Prayers the Prophet (sws) has been reported to have offered in different prayer times. We know that the believers can offer optional prayers. The Prophet (sws) himself offered optional prayers. The number or rak'āt he offered has been taken by the jurists as examples and they recommend that we offer this number of optional raka'āt.
As for the witr prayer it is a part of the tahajjud prayer which was obligatory for the Prophet (sws) and optional for the rest of the ummah. The Prophet (sws) allowed those who found it hard to get up late in the night and offer the tahajjud to offer the prayer even before going to sleep. This was later translated into a separate prayer which is usually termed wājib by the ḥanafī jurists. We believe that these terms and their applications are results of and found in the works of the jurists. They have done a very valuable service to the religion by categorizing everything but we must remain in clear that these terms do not carry any religious sanctity. We will check everything in the light of the Book of God and the Sunnah of the Prophet (sws). The Prophet (sws) it is clear has not rendered the witr obligatory for the ummah. However, once you start offering the tahajjud then witr is its basic component.