Soft washing uses low pressure (generally under 500 PSI) and a professional cleaning solution to kill algae, moss, lichen, and mildew at the root. The chemistry does the work: the water rinses it away. It's the appropriate method for roofs, siding, painted surfaces, wood, and any surface where biological growth has accumulated.
Pressure washing uses high-pressure water (typically 1,500–4,000+ PSI) to remove debris from hard surfaces that can handle the force: concrete, pavers, brick, and hardscape. The two methods are not interchangeable. Using high-pressure water on a roof, for example, typically strips shingle granules and can void manufacturer warranties.