hi ranjeet kumar….
Well i think Buffer selector is used for iteration in our layout mean you can select the particular iteration result through this buffer selector.
As already discussed by Damian Marek in one of the post the buffer selector is a simulation tool to grab certain signals (from the different signal iterations) generated in bidirectional or loop simulations.
As suggested by him you can go through the bidirectional tutorial:
Lesson 5: Bidirectional Simulation — Working with Multiple Iterations
With regards