The actual difference
Pressure washing cleans with mechanical force — the water does the work. Soft washing cleans with chemistry — a solution breaks down the growth, and low-pressure water only carries it away. The pressure at the surface in a soft wash is roughly what comes out of a garden hose.
What belongs to pressure
Hard, flat, non-porous surfaces that are not sealed against water getting behind them: concrete driveways, sidewalks, walkways, patios, garage floors. A surface cleaner is the right tool here, not a narrow wand tip, because it spreads the force evenly and does not leave stripes.
What belongs to soft washing
Anything that can hold water behind it or be scarred by force: vinyl and painted siding, stucco, soffits and fascia, screens, and above all asphalt shingle roofs. Also anything organic-stained, because algae has roots and force only takes the top layer.
The mistake that costs the most
High pressure on siding. It drives water up behind the boards and into the wall cavity, where it does not dry, and it cracks the paint seal on painted board. The damage does not show on the day — it shows as a stain on the inside wall the following winter.
Why soft washing lasts longer
Because it kills the colony instead of removing what you can see of it. Pressure-wash green off a wall and you have taken the visible growth while leaving what it grew from, which is why it can look bad again in a single season. Treat it and the gap before it returns is measured in years.