Hi Shushank,
BERT is an analyzer tool where you can see the result of your digital output from a Photodiode. It could be Q factor, BER value or an Eye Diagram etc. A PRBS generator on the other side is a digital data source and you can not use it as a measurement tool.
Please read the User Manual and Component Instructions manual to read more about the functionality of PRBS Generator and BERT.
Does it help?
Cheers,
Ahmad