Know how to understand your own personal characteristics, preferences, behaviour and traits.
It can be very difficult to evaluate yourself honestly and even more difficult to see yourself as others see you. However, this is what must be attempted. It can come with practice and education. Learn some of the main behavioural traits that are clearly distinctive, see them in others and then ask how much of the trait you may possess or how much others may think you possess and why they may think this.
The knowledge named 'Assessing oneself' is considered to be very important.
In addition, the knowledge is thought to be rather complex and difficult to learn, it is almost always learned from experience, and is specialised knowledge.
Opinion is that it is hardly justifiable knowledge.