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How to understand adding or removing “liquidity” in stock markets with market/non-market orders?
Not all limit orders add liquidity, but all market orders remove liquidity presuming there is liquidity to remove. A liquidity providing order is one that is posted to the limit book. If an order, even a limit order, is filled before being posted to the limit book, it removes liquidity. Liquidity is measured by a balance and abundance of quantities posted on the limit book and the best spread between the lowest ask and the highest bid.