The best description I have as a placeholder ...they reflect 'thinking dispositions', which takes account of the merger of cognitive, emotional, personal and social complexity.
( for those more academically minded, definitions are very hard to come by; a work in progress for me)
My reference is here, a practical focus of how soft skills can be taught:
Claxton, G., Costa, A. L., & Kallick, B. (2016). Hard Thinking about Soft Skills. Educational Leadership, 73(6), 60-64.
Soft skills are implied as 'warm and fuzzy, which undermines their claim to serious attention'.
Soft skills can never be fully 'mastered' like a technical process which can be checked as 'right'.
Soft skills involve the person's 'voice, direct observation and real-time performances', so actually very challenging and possibly more complex than 'hard'. For example, think of technical drawing vs. drawing; one is a simpler subset of the more encompassing whole.