Every company has one or more identification numbers. In the US it's a Tax Registration No., in Canada a BN/NE, in the UK a Company Registration No. and - if the company is vat registered - a VAT Registration No. Similarly, in Australia, you'll get an Australian Business Number (ABN), or a New Zealand Business Number (NZBN) in New Zealand.
When you change the country in your business settings or the country of your client, Billdu displays the appropriate identificators. You will find your business details at Settings - My businesses.
If the ID name doesn't suit your country or the country of your client, you can rename it by clicking on the pencil icon. Rename the ID at your will, press Save and save your business settings. This can be done with your Clients as well.