According to how liquid surge is described for tank vehicles, which loading condition is specifically responsible for causing surge?

49 CFR § 383.119(a)/(b) (causes of cargo surge; stopping techniques when a tank is full, partially filled, or empty); AAMVA CDL Manual §8.2.2, p.8-8-1

Recognizing that surge specifically comes from liquid movement in partially filled tanks helps a driver calibrate how cautious to be on a given run. A load that's mostly empty or completely full behaves differently than one sitting at a partial fill level. This distinction is central to the tank vehicle endorsement knowledge test.