NJ-Supplemental Wages - tax calculation

Hi Experts,

My client posed the following question:

For New Jersey Supplemental wages,especially for Termination Pay, should we be using aggregate or flat rate to calculate taxes?

Sorry, I am not a tax expert and hence donno what to explain them. Can someone plz tell me what the law suggests to use? or is there any where i can go look up?

Plz help....

Thanks a ton,

Ana