When we expressed concentration in percent or ppm or ppb we use

In any mixture Case-I: concentration of one solute total weight with respect to total weight of whole mixture we put w/w.

Liquid in liquid or Powder in powder or granules in granules mixture Case-II: concentration of one solute total volume with respect to total volume of whole mixture we put v/v.

Solid in Liquid Case-III: concentration of one solute total weight with respect to total volume of whole solution (mixture) we put w/v.