A long who is "stopped" receives a warehouse-receipt or shipping certificate and must pay the full invoice-amount — no longer a margined position but an outright purchase. Ownership brings storage bills, insurance and the problem of eventually selling the physical.
Commercial firms do this deliberately. Speculators almost never intend to, which is why brokers force liquidation before first-notice-day. The rare retail trader who is stopped on a crude contract discovers that 1,000 barrels at Cushing cannot be sold with a market order.
Example: stopped on one COMEX gold contract at a settlement of $2,400 means wiring $240,000 against a warehouse receipt for 100 ounces, against perhaps $12,000 of margin the day before.
Related: delivery-notice, retender, invoice-amount, warehouse-receipt, physical-delivery