Limits are expressed in price terms for commodities and in percentage terms for index futures, and they reset each day from settlement-price. Hitting a limit does not always stop trading: in many products you may still trade at or inside the limit, just not beyond it.
Index futures use a tiered structure overnight and then hand over to the cash market's circuit-breaker system during rth.
Example: corn's limit might be 25 cents. With yesterday's settle at $4.50, trading is confined to $4.25-$4.75 all session. A drought forecast can pin it at $4.75 with buyers unfilled.
Related: lock-limit, expanded-limits, circuit-breaker, settlement-price, velocity-logic