CPC1002N - Using as Relay
A mosfet (so, also in the solid state relay) has a substrate diode that conducts when the source potential is higher than the drain potential. So, care should be taken how to connect the "load". 

When you have a SSR made off two mosfets arranged back-to-back with their source pins tied together, their substrate diode are anti-series and it will not matter how the "load" is applied.