What does Selectable Subwoofer Crossover mean ?
What is 9-band?
Does 7.2 means TWO Sub-woofer

In sound reproduction, where different speakers are used to handle different ranges of frequencies (pitches), filters are usually used to ensure that each speaker receives only the frequencies it can handle. Those filters are generally called 'crossover networks', so a 'selectable' crossover means the frequency at which the sound 'crosses over' from the squawker to the woofer is selectable by the user (you). This would make little difference to the average listener, but an audiophile (which you clearly are not) might value this feature.

If you're referring to a feature like an equaliser, this is the number of frequency bands the audio spectrum (generally considered to be 20Hz - 20kHz) is divided into, with a control (boost/cut) for each band. The band centre frequencies usually form a geometric progression rather than a linear division of the whole audio spectrum.

Your assumption is correct. Be aware, though, that many makers selling equipment labelled '7.2' are merely providing a second sub-woofer output in parallel with the first (so they are still only 7.1 channels), whereas a true 7.2 system would have different signals emanating from the two sub-woofer outputs (though there wouldn't be much source material in this format - it's mostly through DSP processing to make the signal difference). Again I would think little advantage to the average listener (over 7.1), but maybe to an audiophile.

Daniel