or complete a tax check where they are renewing an existing licence or applying for an additional licence with another authority.

The Government guidance can be found here for renewal applications:

You must complete a tax check to renew a taxi driver, private hire driver or private hire operator licence, or to apply for the same type of licence with a different licensing authority.

https://www.gov.uk/guidance/complete-a-tax-check-for-a-taxi-private-hire-or-scrap-metal-licence

For new applications:

You must confirm you are aware of your tax responsibilities when you apply for a taxi driver, private hire driver or private hire operator licence for the first time.

https://www.gov.uk/guidance/confirm-your-tax-responsibilities-when-applying-for-a-taxi-private-hire-or-scrap-metal-licence