The AD7714 is a complete analog front end for low-frequency measurement applications. The device accepts low level signals directly from a transducer and...
Datasheet AD7714 on Analog.com
In this case, you are using a single AD7124 and the sync pin is unused. When the SYNC pin is floating, it is considered to be at a high level since the AD7124 has a weak internal pull thus the AD7124 will not reset. The SYNC pin can be floating.