National Emerald Club
National Emerald Club is credit-based rather than spend-based. Short rentals generally earn one credit, while longer rentals divide rental days by four to award additional credits. Free days then become the redeemable currency.
Use Emerald Club free days when cash rates are high because the free day covers base rate and mileage only. Taxes, fees, surcharges, and optional products stay out of pocket, so compare the all-in cash price before redeeming.
If you want Emerald Club free days, do not choose airline or hotel partner earning on that rental because National says those rentals do not earn Emerald Club rental credits. Use Enterprise rentals strategically too, since participating Enterprise rentals can still help with Emerald Club credit earning.