Soft mattresses generally fall within the 3-5/10 range on the firmness scale. They normally use cushioning materials like quilted pillow tops or thick sections of memory foam in their top comfort layers. They are characterized by feelings of deep sinkage, body-contouring, and pressure relief.
You won’t experience too much push back on a soft mattress, which means you’re likely to feel more “in” it than “on” it.