I think this refers to the kind of voice creation that was available with say Tyros 4, where you could import and manipulate waves directly on the keyboard itself.
SX900 is completely consistent with Tyros 5, Genos, S775, S975 in that instead, they use YEM for creating expansion voices from new waves. This runs on a PC and creates a file which is then imported to the keyboard.
All of these keyboards support Voice Set, which is starting with an onboard voice (preset or expansion) and adjusting the basic set of parameters such as volume, cutoff, resonance, attack time etc.