The fundamental principle of international law.

There is only 1 international court under the United Nations (UN) and it is the world court, a.k.a, the international court of justice.  If an international court under the UN can't be legally binding to an external party, an "international court" which is not under the UN can't be legally binding too.

The 2016 PCA ruling against China was illegal because China was not a party and was also not involved in the court proceeding.  A legally binding example would be the Singapore-Malaysia Pedra Branca lighthouse case in which Singapore signed a special agreement with Malaysia to proceed with the ICJ court proceeding on the sovereignty of the lighthouse.

The fundamental principle of international law is that an international court ruling is only legally binding between the parties involved and it cannot be imposed on an external party.

https://www.icj-cij.org/en/frequently-asked-questions

It is stated very clearly that an involved party must have access to the court and have accepted its jurisdiction in any 3 stated ways.  China had not accepted PCA's jurisdiction in 2016 because it was a unilateral PCA proceeding initiated by the Philippines without China's presence.

It is also stated very clearly that the court ruling is only legally binding to the party involved.