What is the significance of LTP and ATP in bracket orders? When to use either of them?

LTP is the last traded price and ATP is the average traded price,

By choosing LTP (last traded price) or ATP (average traded price), you are telling the order to place square off and SL points away from either LTP or ATP.

Just to confirm, LTP you mean the price at which order was executed or the current price of stock shown as LTP in PI terminal market watch?

E.g. Assume PowerGrid stock is trading at 194 as LTP, and I had placed a Bracket Limit Order at 193 and it was executed successfully. Now which LTP price shall bracket order (Square Off Sell and Stop Loss Sell) consider?