I see lot of people ask same question, here is the clarification from Prometric:
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Thank you for contacting Prometric.
Please be informed that each voucher is valid for one exam and may only be used at an Authorized Prometric Testing Center in the country for which it was purchased.
For more clarifications, you may contact the Candidate Care Center in USA at 800-795-3926.Our hours of operation are Monday through Friday 8:00 AM until 8:00 PM Eastern time.
We hope this information is helpful.
Sincerely,
The Webmaster Team/NN
Prometric
Canton Crossing Tower
1501 South Clinton Street
Baltimore, MD 21244
USA
www.prometric.com/contactus/testtakers/globalcsform.htm.