Why can't I complete some fields on my guest's registration?

When registering yourself and a guest, when you reach the guest's registration form, you may see some fields that are unavailable to be interacted with, eg:

These fields have been set by your event administrator to be only be completed by your guest(s). When you continue you will see the following indicator on your registration confirmation page:

When you complete your registration, your guest will receive an email link, which when followed will take them to the guest registration form to complete the remaining fields and confirm their registration.