Two positions are required in ADP to ensure you get paid accurately. When working in a non-PERA school you are subject to Social Security taxes. However, when you are working in a PERA affiliated school you are exempt from Social Security tax but subject to the 11% PERA withholding. ADP does not have the ability to treat some shifts exempt from Social Security or PERA and others non-exempt. For this reason, two positions are needed.