Directory Server

А по существу означает сервер- справочник. Вот определение

When a cardholder makes a purchase at a participating
merchant, a request is sent to the Directory Server to check
if the cardholder is enrolled in the program. The directory
does not contain actual account numbers. Instead, it
contains card ranges with the appropriate Internet address
(URL) to each specific issuer. It is the responsibility of the
issuer to determine cardholder participation

http://www.mastercard.com/us/company/en/docs/SPS_MOAS.pdf