nz serves primarily for Formula Results. Operator expressions will only be evaluated if all operands are available (are not NULL). For example the formula T2-T1 will only be evaluated if the participant has both times T1 and T2. If you would like to evaluate an operator expression when operands are not available you will need the nz function which returns 0 if its parameter does not exist (is NULL). For example in case of net timing, you may want to calculate the net time even if the start time does not exist. For this you can use the following expression:
T2-nz(T1)