Choose +3db frequency

I don't understand what choose +3db frequency in the car icon thing in the graphs mean. And when I look at the details on my graph I don't hit 0 db until 70 hz and I feel like that is really high? Building a sealed box for Stereo Integrity SQL 15 D4. I entered all the ts and electrical specs correctly. I just don't know what I'm doing haha

I believe it's explained here or a simple Google search will explain. Your average characteristics of a car will attenuate low frequency sound. Of course not all cars interiours are equal. I believe attenuation is a ballpark 6dB. So taking that into account you have settings to play with and see how subwoofer will behave. 

It means 3dB above the average of the woofer's response