At the closing bell on expiry day the 'maybe' is over and the bookkeeping is mechanical: U.S. equity options in the money by $0.01 or more are auto-exercised by the clearing house — no phone call, no opt-in — while out-of-the-money contracts simply cease to exist, the buyer's premium fully spent and the seller's fully earned.
Assignment is the mirror of exercise: when a holder exercises, a short position is chosen essentially at random and must honor the contract — deliver shares at the strike on a short call, buy 100 shares at the strike on a short put. It is not a penalty; it is the product the seller was paid for. Sellers who pick strikes they'd genuinely honor turn assignment into logistics, which is the mindset behind the Wheel.
One hazard lives at the strike itself: pin risk. If the stock closes expiry pinned near the strike, holders can still file exercise decisions for a short window after the close, so a seller can go home unsure whether they'll wake up long, short, or flat. The professional habit costs pennies: close or roll a pinned short option before the bell.
One aside completes the vocabulary: American-style options (U.S. stocks and ETFs) can be exercised any day up to expiry, though early exercise is rare because it forfeits remaining extrinsic value — it mostly happens deep in the money or around dividends. European-style options (most index options) exercise only at expiry and settle in cash. The interactive figure runs the clock from 60 days to zero and shows the live probability of finishing in the money — the seller's assignment odds.
Educational, not investment advice.