Volume, liquidity and spread: three numbers, different jobs
Volume counts units traded during a period. Liquidity concerns your ability to transact without an excessive price change. The bid-ask spread is the distance between the best displayed buying and selling prices. They are related, but you need all three descriptions when examining execution.
A visible cost before commissions
Consider a hypothetical quote with a $20.00 bid and $20.10 ask. Buying 100 shares at the ask and immediately selling at the bid would lose $10 before commissions, assuming those quotes and quantities remain available. That is the spread expressed in dollars.
A large daily volume does not tell you how much stock is available at the current ask. Activity might have concentrated around an announcement hours ago. Your order arrives into the current book, rather than the day’s average.
Outside regular hours, participation and available prices can differ. FINRA describes lower liquidity and wider spreads among the risks of extended-hours trading. A convenient time for you may be an expensive time to transact.
What to record
For a simulated transaction, save the bid, ask, intended quantity, session and execution price. Compare the fill with the quote you saw when deciding. Record partial fills separately.
Build this comparison into your market review. It prevents a screen full of active names from being mistaken for proof that every order will execute smoothly.
Finally, compare spread with price. A ten-cent spread on a $2 share is proportionally much larger than the same spread on a $200 share. Use both dollars and percentages before comparing instruments.