This article argues that the notion of "belonging to a Party' to an international armed conflict under Article 4A(2) of the Third Geneva Convention is a necessarily low-threshold requirement. It is submitted that the requirement of "belonging' demands no more than a de facto agreement between a state and an irregular armed group to the effect that the latter will fight on the state's behalf against another state. The article critically examines how the ICTY Appeals Chamber in the Tadi case applied the requirement to "belong' under Article 4A(2) not in order to classify persons, but rather to classify the conflict in the former Yugoslavia as "international'. The Appeals Chamber also considered that the same test should apply for the purpose of attributing state responsibility. It will be argued that there should be no underlying assumption that the same test applies for different purposes. Rather, it is to be expected that different tests developed for different purposes are different. This heterogeneous content of international law does not mean that international law is fragmented. Rather, an argument is made for the application of tests according to their respective purposes.