The term "terra nullius" refers to land that is considered nobody's territory or unclaimed territory. The idea of terra nullius has been historically used in various contexts, including colonialism, but its application to territorial claims in the South China Sea, particularly the Spratly Islands, is not entirely accurate.

China's claims in the South China Sea, including the Spratly Islands, are primarily based on historical assertions and what it perceives as ancient rights. China claims that it has historical evidence of sovereignty over the islands dating back centuries. The concept of terra nullius is not explicitly invoked by China in its claims in the South China Sea.

The United Nations Convention on the Law of the Sea (UNCLOS) is the international legal framework that governs maritime boundaries and rights. The UNCLOS establishes rules for the delimitation of maritime boundaries, exclusive economic zones, and the rights and responsibilities of states in maritime areas. The South China Sea has been a source of territorial disputes among several countries, including China, Vietnam, the Philippines, and others.

China's claims and actions in the South China Sea have been a subject of concern and contention among neighboring countries and the international community. While UNCLOS provides a framework for resolving maritime disputes, not all countries involved in the South China Sea disputes have agreed to submit their claims to international arbitration. China, for example, has not accepted the 2016 ruling by the Permanent Court of Arbitration, which rejected China's historical claims in the South China Sea and upheld the Philippines' rights under UNCLOS.

In summary, China's claims in the South China Sea are based on historical arguments rather than the concept of terra nullius, and the application of these claims to UNCLOS is a complex and disputed issue. The situation is influenced by geopolitical factors, historical claims, and the interpretation of international law.