Selecting the "Document Subject" rename option uses the document subject metadata embedded in the PDF file in the renamed file name.

 

Some documents do not have the "Document Subject" metadata set in the PDF.  If the "Document Subject" metadata is empty, a "-" character is substituted in the renamed file.

 

You can view the PDF metadata fields by opening the PDF in Adobe Acrobat Reader DC and viewing the "Document Properties" under the "File" menu.