As of 2009, SAP requires Maintenance Certificates for all 7.0 and higher based Systems for the Support Pack Installation ( Wartungszertifikate ).
These maintenance certificates can be obtained automatically via your Solution Manager or manually via transaction SLICENSE. SAP created a special license key of type "Maintenance", that is typically valid for 3 months. This one can be obtained in the Service Marketplace and the installed via SLICENSE.
Further information you can find in the PDF:
SAP Maintenance Certificates Information