Chinese diplomats and legal experts have been following developments closely and taking outside opinions, according to sources with knowledge of Beijing's approach to the case.
Some of that work has been handled by the Chinese embassy at The Hague, which has established a formal line of communication with the tribunal, they said.
Tribunal statements and the rules of the case reviewed by Reuters confirm that China can communicate with the tribunal via its ambassador to The Hague, while the court also updates China on procedural moves and opportunities for submissions.
Despite the exchanges, China still planned to reject any decision that favored Manila, the sources said.
Chinese Foreign Ministry spokeswoman Hua Chunying told a regular briefing on Thursday that the "unilateral" arbitration was "a political provocation in the guise of law that seeks to deny China's national sovereignty in the South China Sea".
Charles Jose, the Philippine Foreign Ministry spokesman, disagreed: "The court is a fundamental first step towards a peaceful and rules-based resolution of the issue," he said.
Zha Daojiong, a political scientist at Peking University, said he believed Beijing's strategy of not taking part and then refusing to accept any judgment was set.
"Without China's participation, any ruling can only be an opinion," he added.