In stock trading, S and B represent sell and buy orders respectively. Specifically, S is the abbreviation for "Sale," indicating that an investor has placed a sell order for stocks, while B stands for "Buy," indicating that an investor has placed a buy order for stocks.

These markers typically appear in the order book information of stock trading, helping investors understand the buying and selling activities. When an investor places a buy order at a price higher than the current market price, the order book will display a B marker, indicating a buy-side transaction. Conversely, when an investor places a sell order at a price lower than the current market price, the order book will display an S marker, indicating a sell-side transaction.

Additionally, investors commonly refer to S as the "sell side" or "inside market," and B as the "buy side" or "outside market." By observing changes in the volume of buy and sell orders, investors can analyze the balance of power between buyers and sellers and thus gauge market trends. For example, when S markers appear frequently while B markers are scarce, it may indicate that current investors are more inclined to sell stocks than to buy. Conversely, when B markers appear frequently while S markers are scarce, it may suggest that investors are more willing to buy stocks than to sell.