Soft pens generally refer to calligraphy works created with soft pens, and the theory should include brush and soft pens. But in fact, there is no such thing as soft pen calligraphy. There has always been only a distinction between hard pen calligraphy and brush calligraphy. When you know it, you don't have to go into it.
Hard pen can be practiced as a whole, which is relatively easier than brush pen. If you want to practice well, you have to practice stroke by stroke. In modern times, hard pen calligraphy was born. Relatively speaking, we call it brush calligraphy in ancient times. Strictly speaking, there is no so-called soft pen calligraphy. Moreover, the soul of calligraphy is interlinked, not hard pen or brush pen.