What is a "recordIdentifier" with a ppn prefix in the SRU response?

This "recordIdentifier" is the German National Library’s internal identification number. It consists of the namespace “ppn” and the identification number in the format "ppn:940116162".
Explanation: The PPN/IDN is delivered in MARC field 001, for example 129602434.
As the interface is generally oriented, the SRU response always returns the German National Library's internal identification number – no matter which identifier was sent in the SRU request.