The set of two jumpers will probably have jumpers connected to both the jumper positions. These are to be removed in case the crossover is to be used in a bi-amplification setup. i.e. both pairs of input terminals to be used, and connected to two different amplification channels. If these jumpers are not removed and bi-amplification connections are made, you could blow the amp(s).
The set with three jumper positions is the tweeter attenuation section. Of this the topmost should be the -3dB position, the middle will be the 0dB position and the lowest will be the +3dB position. So depending on which of the positions the jumper is placed in, the tweeter will be at -3, 0 or +3dB with respect to the midwoofer.
I believe the crossover frequency for the set is something in the range of 3.5khz 6dB/ octave.
