Possible Clues:

• Songs that are rarely hits
• Unlikely-to-be-fulfilled concert requests
• They're opposite hits
• Elvis's 'Hound Dog' and 'Anyway You Want Me'
• Partners for singles too devoid of energy
• Lesser numbers also lacking energy
• Some deep cuts
• 'P.S. I Love You' and 'Revolution,' e.g.
• Singles may offer these as well, it's said
• Lesser hit locations