Demurrage and detention: the paperwork that starts the clock
The definitions are short. Demurrage is charged while a container sits inside the terminal beyond its free time. Detention is charged once you have taken the container out and are still holding it. Demurrage is about the port's space. Detention is about the carrier's container.
The more useful question is why the free time runs out. It is usually not congestion. It is usually a document.
Free time is shorter than it sounds
A typical allowance at destination is three to seven days. That sounds generous until you list what has to happen in that time: the arrival notice is issued, the entry is filed and released, the delivery order is obtained, the haulier is booked, and a slot is available. If any one of those stalls, you lose a day. Days at demurrage rates are not cheap.
The clock does not care whose fault it is.
The document failures that cost days
A late or wrong ISF. For ocean cargo bound for the US, the filing is due 24 hours before loading at the foreign port. A shipment that arrives without one can be flagged for examination on arrival. An examination takes days, and all of them count against your free time.
No telex release, and the originals still in a courier bag. The cargo cannot be collected until a bill of lading is surrendered somewhere. If the originals are still in transit and the vessel was faster, the container waits for the post.
A certificate of origin that has not been legalised. Where the destination requires consular legalisation, the chain of chamber stamp and then consulate can take longer than the voyage. If you start it after shipment, you have started too late.
Consignee name different on the invoice and the bill of lading. "Nordwind Import GmbH" on one and "Nordwind Importe GmbH" on the other is enough for a broker to stop and ask. Asking takes a day, even when the answer is obvious.
A goods description nobody can classify. The broker cannot file an entry against "Item 4471-B". They will email you, you will reply, and that is another day.
The clock does not stop while you sort it out
People find this unfair, but it is important to understand. Free time is a contractual allowance, not a fairness rule. It keeps running while customs examines the goods, while your broker waits for a corrected document, and while a consulate takes its time. The carrier does not have to pause it because the delay was not your fault.
Some carriers will consider a waiver for a delay clearly caused by the carrier. Almost none will for a delay caused by paperwork.
Where to spend the effort
Everything above is decided before the vessel sails. The consignee name, the description, the ISF, the release method, the legalisation. All of them are set at origin, and all of them are cheap to get right at that stage.
Check the document set once before shipment. Read the invoice, the packing list and the bill of lading side by side. Ask whether they describe one consignment in the same words. That review takes an hour. A week of demurrage on one forty-foot container costs much more.