A market order is an instruction to purchase a certain quantity of a security regardless of the price at which it is available. Market orders are restricted for trade-to-trade and debt category instruments since they are typically illiquid scrips.

Due to a lack of liquidity, the bid and ask spread in the scrip is quite wide, which can have an immediate negative impact on your P&L. The bid/ask price may differ significantly from the instrument's last traded or theoretical price.
Note:

1. All non-EQ category instruments have their market orders banned. In this post, you'll learn more about the various categories.
2. Only if there is liquidity will an order be executed once it has been placed.