"Li Ming" usually refers to the understanding and observance of etiquette. Including respect for others, understanding of what you don't want others to do to you, and understanding of bring disgrace to oneself's social communication rules. To put it simply, "courtesy" means knowing how to communicate with people in an appropriate way, how to respect others and yourself.
"Understanding" focuses more on the insight and understanding of truth and things. This includes understanding the principle of "don't do to others what you don't want others to do to you", understanding that respecting others means respecting your own ideas, understanding the common sense of looking light and insulting others, and understanding etiquette is the foundation of doing things. In other words, "understanding" means understanding the basic principles of communication with people, thus guiding your behavior in real life.
To sum up, "courtesy" emphasizes social skills and respect for others, while "understanding" emphasizes insight and understanding of basic principles. Although the emphases of the two overlap, they are not exactly the same.