hi ranjeet,
In the recent past, the component functioned in the following fashion. Setting the Number of FFT points sets the total number of available (not necessarily used) sub-carrier positions centered at DC or 0 Hz. The number of sub-carriers would then set the total number of sub-carriers actually used (for example 4 sub-carriers might be used for a QPSK format). The position array then sets the starting point of the range of the sub-carriers used. You can refer to the link provided by alistu for further refrence.
Thanks and regards