The master and slave relationship between the chassis synchronized by IEEE 1588 Synchronization is determined by the IEEE 1588's Best Master Clock algorithm. It will choose the highest quality clock on the network as master for the chassis synchronization purpose.
To configure the master and slave relationship manually between chassis synchronized by IEEE 1588 Synchronization, you can adjust the Priority 1 and Priority 2 values demonstrated in the image below via Test Panel of the PXI-8863H in NI MAX.
It should be noted that the smaller the value of Priority 1 and Priority 2 of a chassis, the higher the priority is given to the chassis. Hence it is recommended to assign value between 1 and 10 to the chassis you wish to configure as master.