Nutrient Completeness score is a rough measurement of the reliability of your micronutrient reporting on any given day. Nutrient Completeness Score is calculated for each micronutrient, and it gives you an indication of how many foods logged contained information for this micronutrient.
If you see that you are not hitting your target for the day, take a look at the Nutrient Completeness Score. If it is not 100%, it is possible that your micronutrient intake is being underreported because the food entries you logged were missing data for this nutrient.