not across them

[idiotmike]

Dear teachers,

"China now allows yuan to circulate freely outside its borders, but not across them."

Could you help me understand what "but not across them" means? Thank you.

[5jj]

If you read the whole paragrapg from The Economost, the meaning becomes a little clearer.

"According to the Financial Times, Wang Qishan, a Chinese vice-premier, was due to give China’s official blessing to London’s efforts during a visit to Britain this week. The offshore business is still in its infancy, but is growing fast. China now allows yuan to circulate freely outside its borders, but not across them. Foreigners must earn yuan by selling goods to China or, in some cases, direct stakes in their companies. They can deposit these yuan in banks offshore, or invest them in offshore bonds, but they cannot invest them on the mainland without the government’s permission."