When using a formula, you can assign another formula as a variable. This is similar to using columns as variables, except that the formula doesn't have to be in a column already - you can enter the formula exactly as you want it to work for the new variable.
To create a nested formula, simply select Formula... in the attribute selection drop-down. A new Formula field will appear for the variable.
Once you finish setting up a nested formula, you can collapse the dialog by clicking < Back to Variable List. To edit the nested formula later, simply select the variable from the Variables list.
Variables in Nested Formulas
Nested formulas can have their own variables.
Variables in a nested formula are not the same as the variables declared by a parent formula. Variables do not overwrite each other, even if they have the same name.
Unlimited Nesting Levels
Nested formulas can also use formulas as variables. Doing this, you can create very complicated formulas that rely on several levels of nested formulas.
There is no nesting level limit.
A word of caution - the more nested formulas you include, the more difficult it becomes to troubleshoot the column.