The stocks that fall under the trade-to-trade settlement system of the exchange come under this category.
Each trade here is seen as a separate transaction and there’s no netting-out of trades as in the normal rolling system.
The trader needs to pay to take delivery for his/her buys and deliver shares for his/her sells, both on the second day following the trade day (T+2). For example, assume you bought 100 shares of‘T’ grade scrip and sold another 100 of it on the same day. Then, for the
shares you have bought, you would have to pay the exchange in two days. As for the other bunch that you sold, you should deliver the
shares by T+2 days, for the exchange to deliver it to the one who bought it.
Failure to produce delivery shares against the sale made would be considered as short sales. The exchange will, in that case, on the
T+3rd day, debit an amount that is 20 per cent higher than the scrip’s closing price that day. This means unless the scrip’s price falls
more than 20 per cent from the price of your sale transaction, you would have to pay a penalty for the short sale so made.
Even so, there will be no credit made to you in the case of substantial fall in the share price. The exchange will, instead, credit the gain to its investor fund.
Stocks are regularly moved in and out of trade-to-trade settlement depending on the speculative interest that governs them.
http://www.bseindia.com/datalibrary/disp.asp?flag=T