What does "6 V stop-start applications" mean?
What it actually says is "... is an ultra-efficient, quad-channel Class-D audio amplifier that supports the 6 V requirement for start-stop vehicles."

Vehicles that stop and restart their engines for short periods of time (e.g., at stoplights) can experience power bus sags. What this brochure is saying is that this chip will be well-behaved even if the power supply sags as low as 6V.