BEIJING — As China moves to start enforcing a new cybersecurity law, foreign companies face a major problem: They know very little about it.

The law — which was rubber-stamped by the country’s Parliament last year — is part of wide-ranging efforts by Beijing to manage the internet within China’s borders. Those efforts have been stepped up in the years since Edward J. Snowden, the whistle-blower and former American intelligence contractor, revealed that foreign technology firms could help governments spy.

And while Chinese officials say the new rules will help guard against cyberattacks and prevent terrorism, critics, many of them from businesses, have their concerns. Companies worry that parts of the new law, which takes effect on Thursday, will make their operations in China less secure or more expensive. In some cases, they argue, it could keep them out entirely.

The law will have a big impact on how business is done in China, said Michael Chang, an executive with the Finnish technology company Nokia and the vice president of the European Union Chamber of Commerce in China. But, he said, “There’s unfortunately a lot of confusion.”