Yi supports; Ji asks for caution
The simplest definition is clear: Yi marks activities that a date traditionally supports; Ji marks activities that a date traditionally cautions against. The harder part is reading the activity correctly. Yi for one kind of ceremony points to that activity category. Ji for one serious matter marks where the day asks for caution, preparation, simplification, or another timing choice.
Yi/Ji belongs to the Huang Li record. Everyone looking at the same date sees the same date labels, while a personal reading may explain how those labels meet a person's current focus. Birth data can change interpretation while the almanac record remains the shared date record.
In modern use, the most important skill is translation. A classical entry often points to a category of action rather than a perfect modern word. The reader has to ask whether the actual matter is ceremonial, contractual, domestic, commercial, medical, legal, social, or simply routine.
