Final answer:
Lawyers referring to a past decided case in a civil lawsuit are operating under a common law system that values legal precedent.
Step-by-step explanation:
In a civil lawsuit, where lawyers refer to a similar case that has already been decided, they are operating within a common law legal system. This system relies on the principle of legal precedent, where past court decisions influence future cases. The common law system is different from civil law systems, which are more based on codes and statutes, and from international law, which governs the conduct between sovereign states. Moreover, this type of legal referencing is distinct from criminal law systems, which deal with crimes against the state or society, not individual or private disputes.