BTST stands for “Buy Today & Sell Tomorrow” and as the name goes, it is applicable when an Investor buys the stock today which is T day and sells it the next day which is a T+1 day.
Please note*while selling this stock you actually don’t have it because the stock is deposited only on T+2 day. So here you are expecting the stock to be deposited in your account on T+2 day but in case the stocks are not deposited then it will lead to the auction.