Indices like BSE Sensex or NSE Nifty cannot have any volume as they are not tradeable instruments. Volume is basically the number of trades (1 Buy and 1 Sell is 1 Volume) that have taken place for each candle. Volume data is available only for tradeable instruments like Index Futures. So the BSE volume you are referring to is actually SENSEX Futures volume. You may see the same volume for NSE Nifty Futures.
Sandarpan Mukherjee
Aspiring trader