Crossover slopes are the rate that the frequency will drop above or below the crossover point.
12 dB slope will drop 12 dB per octave.
From 20 Hz to 40 Hz is one octave. 40 to 80 Hz is another octave. 80 to 160 is another and so on... So basically upwards you double and downwards you cut in half.
Now,
At 12 dB, the crossover will start to reduce volume 12 dB per octave.
At 24 dB, the crossover will start to reduce volume 24 dB per octave.
Use the 12 dB slope if you want to get a gradual reduction, and still be able to hear some of the frequencies past the crossover point.
Or,
Use the 24 dB slope if you want a sharp reduction of frequencies past the crossover point.
Personally I would use the 24 dB slope because it protects the mid-bass driver from bottoming out at low frequencies.
