What is the Area Code on a Domain For?

The area code box on a domain is not what you'd think it's for.

The area code box on a domain is used for certain carriers in other countries.

However, it is not used in the UK, therefore this field's function has no use unless you're outside the UK.

Because the field is required, you can just enter a "0", or some other number of your choosing. What you choose will have no affect.