Returned when either of the two limits on this endpoint is exceeded: the record limit, which meters the number of records in the request body, or the request limit, which counts requests regardless of their contents.
In both cases no records were accepted and nothing was imported, so the same body can be resubmitted unchanged.

Record limit: 3000 records per minute per organization with a burst of 5000 records, and 4800 records per minute per holding with a burst of 8000. A request may carry at most 100 records.
The burst refills continuously at the sustained rate, so a client that has been idle can submit up to the burst at once and averages the sustained rate thereafter; from a full burst the first minute admits about 8000 records for a single organization.
Organizations that belong to a holding are metered against both allowances and limited by whichever binds first.
A rejection by the record limit carries a Retry-After header.

Request limit: 500 requests per minute per organization and 2,500 requests per minute per holding, counted regardless of their contents.
A rejection by the request limit carries no Retry-After header, so clients should retry with their own backoff interval.

Both sets of values are standard and may be adjusted per environment, so pace against the Retry-After header where it is returned rather than hard-coding them.