HE means high efficiency in markets that use that designation. It can apply to front-load or top-load washers; by itself, it does not identify an energy label or a specific drum geometry.
What changes in the way it washes
HE designs combine mechanical action and water management to clean with a smaller bath than comparable conventional designs. The machine may wet the load in stages, spin it to estimate it, and add water after the clothes absorb part of the fill. A low visible water level does not prove the appliance is malfunctioning.
The detergent must be compatible with the system. Too much suds takes up space in the system and can affect rinsing or trigger cycle adjustments; adding more water or product because of the drum's appearance is not a reliable way to adjust the wash. The selection of the compatible detergent should follow the manual and the packaging.
HE does not replace measured data
A basket without an agitator column may have an impeller, but that feature alone is not enough to classify it as HE. It also does not let you infer its energy class. ENERGY STAR, for example, publishes criteria and data for certified equipment within its own program; that is different from a general high-efficiency designation.
To compare two machines, consult the labeling scheme and the water used per cycle, under the same capacity and cycle conditions. If a machine suddenly changes the way it fills, do not excuse it just because HE appears on the housing: record the change and check how it is working.
Use HE to recognize usage requirements; use equivalent test data to compare consumption.