the X1 and X10 are multipliers. multiply the frequency written on the amp dial by whatever you have it set on, and thats the crossover frequency. just allows you to use as HPF or LPF.
the 12db and 24 db are slopes. 24 being steeper, 12 being more gradual. this is the rate at which the crossover frequency will "roll off". and off means your not using the amp crossover, full range.