Why is a -50dBm S/N ratio considered a "perfect signal"? Edit: dBm as a unit for SNR doesn't make sense, which is why I was confused. The Y axis is overall si

Why is a -50dBm S/N ratio considered a "perfect signal"? Edit: dBm as a unit for SNR doesn't make sense, which is why I was confused. The Y axis is overall signal strength, not S/N ratio.