You should not normally need to edit assignment set formulas.
If you enter multiple criteria to define an assignment set, the conditions that are generated are joined by AND or OR. You might want to edit the formula to change the brackets in the generated conditions. This changes the order in which the conditions are tested.
To view an assignment set formula, query it in the Formula window. The formula type is Assignment Set and the formula name is the same as the assignment set name. To edit an assignment set formula, make a copy, as for a QuickPaint formula.
Using the Formula Window
Creating an Assignment Set