What is the difference between "fard" (obligatory) and "wajib" (required) according to the Hanbali school? And does forgetting to say the basmalah (in the name of Allah) invalidate wudu (ablution), given that it is considered wajib in the Hanbali madhhab?

Praise be to Allah, and prayers and peace be upon the Messenger of Allah, and upon his family and companions. As for what follows:\n\nNaming in ablution is obligatory according to the Hanbali school in the school's reliable opinion, and it is excused if forgotten. According to Imam Ahmad, there is a report similar to the majority that it is a Sunnah.\n\nAs for the difference between obligatory and mandatory, the Hanbalis, like the majority, did not differentiate between them in essence; both are what Allah has obligated upon the legally accountable. However, the Hanafis distinguished between them: the obligatory is what is established by definitive evidence, while the mandatory is what is established by speculative evidence.\n\nIn certain branches, the Hanbalis distinguished between them in places, making the obligatory more emphasized than the mandatory, such as the obligations of prayer and Hajj that cannot be compensated, unlike the obligations that can be compensated by prostration of forgetfulness or blood.\n\nThe narration about Ahmad varied in some issues like the charity of breaking the fast, rinsing the mouth, and inhaling water in terms of differentiating between them being obligatory or mandatory."