Before a slot can be booked
Three things must be true: the container is discharged and available in the terminal system, all holds are cleared (customs, biosecurity, shipping line), and the line has issued a release. Any one of these outstanding blocks the booking.
Biosecurity holds are the least predictable. If your cargo is in a category likely to be inspected — timber packaging, natural fibres, machinery with soil risk — build extra days into the plan.