Short answers, pulled from the story.
The International Law Commission listed treaties, national legislation, diplomatic correspondence, and opinions from legal advisors to governments as forms of evidence in 1950. This list aimed to identify state practice in a world without a single global police force.
The court emphasized this distinction sharply in the North Sea Continental Shelf case of 1969. Judges noted that acts motivated by convenience or tradition fail to establish custom under international law.
Prohibitions on genocide, slavery, torture, and war of aggression fall into the category of peremptory norms. These fundamental rules allow no exceptions regardless of how many countries agree otherwise.
Article 38(1)(b) defines custom as general practice accepted as law. The court confirmed this definition in the Legality of the Threat or Use of Nuclear Weapons advisory opinion of 1996.
The laws of war existed as unwritten customs before becoming codified in the Hague Conventions of 1899. Later treaties like the Geneva Conventions formalized many existing rules but did not cover all legal matters arising during conflict.