It represents a numeric expression of client´s payment morale. The value means a sum of positive days, if invoice is paid prior to due date, and a sum of negative days, if invoice is paid after due date. It is a so-called total scoring. For instance, if a client pays 2 days prior to due date each month for 10 months, his scoring is 2+2+2+2+2+2+2+2+2+2=20. And vice versa, if he pays 2 days after due date, his scoring is -20.
You may treat your client based on scoring number - clients with positive scoring may receive more attention and quicker response to their requirements, and vice versa.