Earlier quoted context omitted.
They arbitrate part of the hague conventions. the hague conventions make up part of international law. It's a court of arbitration, your comparison to the UN is apples to oranges, in fact, the UN isn't really involved in international law setting.
I am not comparing it to the UN, but how it is referred to in the media is often misleading, especially the "in the Hague" part. Quite a few articles even dropped that bit and just referred to it as an "UN court/tribunal". It gave the impression that China is defying the UN, not saying that China wouldn't do something like that, but they didn't in this instance. And because it is a court of arbitration, both sides ha…
You're right that they _would_ do something like that! ;) In fact, they did in this case. I'll explain below.
>And because it is a court of arbitration, both sides have to agree to have their dispute resolved by arbitration in the first place. China did not agree to it.
Not quite. China has ratified the UN Convention on the Law of the Sea (UNCLOS) [0], which designates the Permanent Court of Arbitration in the Hague [1] as the default means of dispute resolution between parties. China knew this going in, and agreed to it when they signed the treaty. This court ruled against China on the matter of the Nine-Dash Line [2]. By defying the court's ruling, they defy the UNCLOS, and thus the UN.
You're technically right that in the case of the Nine-Dash Line, China did not agree to participate in arbitration. However, arbitration was allowed to proceed without their cooperation according to the rules of the UNCLOS, which China had already signed. Treaty law doesn't only apply when it's convenient.
[0] https://pca-cpa.org/en/services/arbitration-services/unclos/ [1] https://en.wikipedia.org/wiki/Permanent_Court_of_Arbitration... [2] https://en.wikipedia.org/wiki/Nine-Dash_Line#Arbitral_tribun...