It stands for Key Scale Panning. You will also see this as KSP in Yamaha documentation. When you select a KSP voice the voice will sound like it's coming from the left or right depending on which key you're playing-- e.g., C1 is toward the left and C6 is toward the right. The KSP voices are XG voices that use BSMSB 0 and BSLSB 1.

This is all I could find. If you look at the top of the table for the voices in the manual it says Key Scale Panning above the column of voices that end in K.

My wife asked if I had seen the dog bowl. I told her I didn't even know he could.