It means that polyurethane or some other stabilizing substance is drawn into the piece by vacuum. This can be done in block form or after it is split, but I see that often the scales will warp when curing and you'll have to flatten them again. The vacuum method is best, but some will soak it in a thin stabilizer after the scales are 95 percent shaped, effectively creating a layer in the outer depths of the piece.
Ideally I want stabilized wood in block form and I will split it after the stabilization.