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 There are actually two system variables for the part path, although both hold the same value.

XML_DOC_PATH
PART_PATH
 
Sample GPP2 code can look like this:
 
SYSTEM_CALL "move " DBL_QUOTE OUTPUT_DIR FILE_NAME DBL_QUOTE " " DBL_QUOTE PART_PATH "\" FILE_NAME DBL_QUOTE ;                           
OUTPUT_DIR = PART_PATH + "\" ;

 
Note that DBL_QUOTE has been added. This is to put both strings, current and destination, inside double quotes. Without these, if you have spaces in the names the system command breaks and fails because the file is not found.
Notice also the “\”  added to the line where the OUTPUT_DIR is updated. Without it your editor will not find the file to open at end.