Where is the documentation for this iOS companion call?

Configuration
1

What does volume mean with this iOS device call? If crtitical is set, then 1.0 is supposed to mean 100%. I can’t find any documentation for this.

        data:
          push:
            sound:
              name: 3rd_Party_Critical_Haptic.caf
              critical: 0
              volume: 1
2