National Laws are rules that are appropriate for a particular country, merchants and trades regulations to promote justice. They are established by convention and custom, and are not directly related to Biblical rulings.
There is an opinion that there should not be one Code of Law for Noahides, but perhaps 'seventy' codes of law for each of the seventy nations, where each Code of Law would reflect the national character of that nation. In this case, the details of the decisions -- particularly where they concern the third type of Noahide law above -- may be different from nation to nation.
Rabbi Yaakov Anatoli (1194-1256) in HaMelamed put it this way: When the Noahites were enjoined concerning Justice, they were put under obligation to create legal arrangements .... It is incumbent on the judges to draw up rules of equity that shall be appropriate for that particular country, as exemplified by the manner in which this matter is handled currently by the nations, severally. Likewise, it is incumbent upon merchants and upon the members of the trades to establish regulations for themselves... and whatever emerges as the law in this manner is law, as much as that which is written in the Bible. Furthermore, anyone violating this law violates Scripture, because Scripture commands the individual to accept the decisions of the contemporary jurists. The dictum, "The law of the land is the Law," relates to this concept.
- Quoted by Reuben Margolioth, Margolioth Hayarn. Jerusalem: Mosad Harav Kook, 1958, volume 11, page 20. (Sanhedrin 56b, section 9.)