First, what is liquidity?
Liquidity is a financial term that’s used to describe how easily you can convert a security to cash without substantially affecting its market price. This usually depends on how many shares are available at any given time for the purchase or sale of a specific security.
Stocks or ETFs with low liquidity and fewer shares trading (low volume) may have wider Bid-Ask spreads, and investors may not be able to buy or sell at their desired prices. Securities with high liquidity and a high volume of shares being traded will generally have tight Bid-Ask spreads that are only a few cents (or less) apart (depending on the price of the security).
As a result, ECNs are always promoting higher liquidity to encourage an efficient market for all participants. When you place a trade to buy or sell shares that removes liquidity from the market, there is sometimes a small per share fee charged.
Removing vs. adding liquidity
An order is generally considered to be removing liquidity from the market if it fills immediately.
Market orders which fill (or execute) immediately are always considered to be removing liquidity since your order basically “jumps the queue” of the other limit orders currently sitting on the exchange’s order book.
Marketable limit orders (limit orders above the Bid for buys, or below the Ask for sells) are also considered to be removing liquidity as they’re executed immediately just like regular market orders.
What is an example of adding liquidity?
If you place a non-marketable limit order that requires your order to “sit in the queue” or on the exchange’s order books to be filled, it’s considered to be adding liquidity.
This would be a limit order where the limit price is lower than the Ask (when buying), or higher than the Bid (when selling).