WHY WE WON
It seems that we can boil down the legal conclusion of the arbitral ruling as based on only two key questions.
One, what is an island? A rock is not an island. So there. Even if China builds up a rock, it still cannot become an island. It may be a feeler island, but still that’s not an island in the legal contemplation of international law. UNCLOS says that artificial islands are not properly islands. (Otherwise, any oil rig can claim to be an island!)
And there’s the economics of islands. Under UNCLOS (Art. 121(3)), only natural formations that can sustain economic life on its own, can have maritime zones, such as an Exclusive Economic Zone (EEZ).
Two, what is an archipelagic state? It is one composed of many islands. PH is archipelagic. China is not. It appears that China conceded its status as a coastal (non-archipelagic) state when it signed the UNCLOS because the UNCLOS classifies countries in only two ways — archipelagic or coastal. The Tribunal ruling, in para. 573, categorically restates that China is a coastal state.
The answers to these two questions determine the EEZ, which is 200 miles from the coastline of a non-archipelagic state; and 200 miles from the archipelagic baseline of an archipelagic state. The baseline is a point-to-point boundary that encompasses or includes the islands of an archipelagic state.
An outlying island in the South China Sea, even if it could be claimed as territory of China, cannot result in an expanded EEZ based on archipelagic baselines because China is not an archipelagic state. (Consider the following related question: Can the US consider the waters between Hawaii and Los Angeles as “internal waters” using the archipelagic baseline approach? The answer is in the negative because, like China, the US is not an archipelago.)
The EEZ of an outlying island is 200 miles around that island because such an island is treated like any other land territory (Art. 121(2)). The EEZ cannot extend beyond the 200 miles (beyond this, there would be continental shelf (something else) or open international waters (high seas)). And an EEZ could be delimited if competing EEZ’s from other nearby states exist.
The ruling states that the disputed territories are not at all islands, and therefore cannot provide China an EEZ. Practically all the major disputed territories are inside the EEZ of the Philippines because it is an archipelagic state. Therefore, even ‘rocks’ can be useful to the Philippines, if they are located within the EEZ based on the archipelagic baselines of the Philippines. These useful rocks include Scarborough Shoal, Second Thomas Shoal, Johnson Reef, McKennan Reef, Hughes Reef, and Mischief Reef. Some of the disputed rocks are outsize the EEZ of the Philippines.
An important caveat is that territory and sovereignty are matters of international law not subject to the UNCLOS. Nonetheless, disputes relating to the EEZ are pointedly the subject of the UNCLOS.
WHY IT MATTERS
EEZ means maritime zones (not necessarily territory in the usual ‘conquest’ or ‘historic’ sense) that we can, under international law and UNCLOS, consider as usable only by us. We can, by negotiation, lease or allow others to use the EEZ, but the extent to which the Philippine Executive can do this is governed by the 1987 Constitution.
NB: The map of PH EEZ is from Wikipedia.