I don't know what you're talking about. Both of those exist.

Zero volume is when a speaker's diaphragm is still. On an 8-bit PCM audio file, it corresponds to a value of zero.

In the context of a signal, full volume corresponds to the 127 value in an 8-but PCM audio file (or arguably -128). In the context of a speaker, those values should push the diaphragm no further than how far it can travel linearly without distortion. Obviously the user may want to turn down the volume from this full volume.

I hope you understand that even when using an audio editor that displays values in dB, the underlying values are integers (or floats) that absolutely have zero and "full volume" meanings, and conventionally map respectively to -∞ dB and 0 dB.