Correct. For SPL you get your sub, box, and car to peak all at the same frequency. This can be done by tuning slightly below your cars resonance, or by bringing your cars resonance down closer to your tuning frequency via mass loading panels with sound dampening.
Resonance can be roughly found by doing a frequency sweep with a sealed box in your car to see what the peak is.
For daily, its actually nice to port much lower than your cars resonance so you get two gain peaks; one from the box and one from the car. With a little EQ help, you can then get a very flat response.