In the Rx signal chain there is an external LO used which is connected to /2 block . ( This was documented on below mentioned support file : AD961RFandBBPLLUserGuide2.4.pdf)
The initial state of this block is not mentioned (no register info /value).
Customer is using BPSK modulation technique in its application which is very sensitive to phase variation.
They want to synchronize the base band signal with the LO freq signal. And they want to know if /2 block will cause any phase variation at the startup.