According to Indian Railways (scroll to the bottom)
CHANGE OF BOARDING POINT : If a passenger desires to occupy reserved
accommodation from any station enroute, he would be permitted to
entrain at any intermediate station he chooses irrespective of the
distance from the starting station subject to the following conditions
A specific request must be made in writing at any computerized reservation counter and reservation made atleast 24 hours before the
scheduled departure of the train from the starting station.
The Railway Administration reserves the right to use such accommodation from the originating station up to the station at which
the passenger is due to entrain.
No refund will be permissible for the portion of journey not performed by the passenger.
As per rules the seat allotment is based on your boarding station.