Ø...

You mean ∅ ? ∅ means when there is no avaible value in a group in other words,an empty group is showed with ∅ in math. For example: Group A=numbers beetween 1 and 4 (2&3) Group B=numbers thats higher than 6 (7,8,9,10 . . .) values in intersection of group A and group B=∅ (none)