[bazar01]

You probably meant that the system needs to be evacuated or a vacuum pulled to remove air and moisture. This is a standard procedure after the leak has been identified and fixed whenever a system has completely leaked the refrigerant or the system has been opened.
Pumping down is used to collect refrigerant inside the condensing unit by closing the high side service valve and using the compressor to pull all the refrigerant and store it inside the condensing unit on most residential split ac units.