We use the Australian version of ZohoOne & Books.

Currently their is a field for the company tax ID, which in Australia is the ABN.  This field is validated to 11 numeric digits as per the rules in Australia.

When exporting to foreign customers, this field is no longer useful because of the restrictions.  Upon contacting support, they identified that Zoho Books does not have a field for customer Tax ID and that a custom field needs to be created.  

Suggestion : Add a Customer Tax ID, and Company ID (government ID) fields (In addition to the ABN field) to allow for standard Tax ID or Company ID identifiers to be recorded and used on Sales orders/Sales Invoices.  In many countries it is mandatory to include the Tax ID on the invoice for customs purposes.

This is normally 'standard functionality' in other ERP platforms and it should not be a 'custom field'.