Information below is specific for the United States, but also holds for other countries outside of the EU. For other countries the names of the required ID will likely vary.
The unique number required by a Parcel carrier for identification by US customs is typically the Employer Identification Number (EIN) or the Tax Identification Number (TIN) of the business that is sending or receiving the parcel. These numbers are assigned by the Internal Revenue Service (IRS) and are used to identify businesses for tax and regulatory purposes. In the case of international shipping, the EIN or TIN is used by customs officials to ensure that the goods being shipped are properly declared and that all applicable duties and taxes are paid.
For private individuals:
If the recipient of a parcel is a private individual rather than a business, the unique number that is typically used for identification by US Customs is the recipient’s Tax Identification Number (TIN) or Social Security Number (SSN). The TIN is issued by the IRS for tax purposes, while the SSN is a unique nine-digit number issued by the Social Security Administration to US citizens, permanent residents, and temporary residents who are authorized to work in the US.
In general, a private individual receiving a parcel from overseas may be required to provide their TIN or SSN to the carrier or customs officials in order to clear the package through customs and pay any applicable duties or taxes. However, the specific requirements and procedures can vary depending on the type of goods being shipped, the value of the shipment, and other factors.
What happens when you ship without providing this number?
Your shipment will be blocked when it arrives at customs until you provide the number to the carrier. If you do not provide it, the shipment will be returned to sender. The example below shows the tracking screen of a blocked / on hold shipment for an arc generator shipment of which the customer provided the wrong number.