What is the explanation of the hadith: "Abusing a Muslim is Fusuq (disobedience) and fighting him is Kufr (disbelief)"?

Reviling means cursing, which is to mention about another person what is in them and what is not in them. As for fusūq (transgression/wickedness), it is departing from obedience to Allah and His Messenger, and it is more severe than mere disobedience. Fighting is waging war. The kufr (disbelief) mentioned in the hadith, "Reviling a Muslim is fusūq, and fighting him is kufr," is kufr lesser than major kufr, and it is not the kufr that expels one from the fold of Islam. Al-Nawawi mentioned in his commentary on Sahih Muslim that reviling a Muslim is haram (forbidden) by the ijmāʿ (consensus) of the Ummah, and its perpetrator is a fāsiq (transgressor). Fighting a Muslim without right is not considered kufr that expels one from the fold of Islam unless it is deemed permissible. It was said in the interpretation of the hadith that it refers to someone who deems it permissible, or that what is meant is kufr of kindness and blessings, or that it leads to kufr due to its ill omen, or that it is like the action of disbelievers.