The interbank association is responsible for approving medium-term notes and bills sold by companies, and most of those securities are held by banks.

WSJ: China Tightens Rules on Bond Issuance

Companies put money in short term Treasury notes and bills to settle trade payments.

FORBES: Is China's Ownership Of U.S. Debt A National Security Threat?

The railway ministry, which is China's largest corporate-bond issuer, used all of last year's quota for long-dated bonds and also raised an additional 50 billion yuan via short-term bills and medium-term notes.

WSJ: China's Railway Ministry Gets Approval for Bonds