Every field in ACF has a field key. It is unique for every field created and remains the same. The reason for this is that field names can be duplicated. ACF uses the field keys in instead of the names in all of it’s forms. When editing a field group you can see the field keys if you click on Screen Options in the upper right and then check Field Keys.
The validation for the ACF field is included on the user registration form, but you have to build your own validation filter. https://www.advancedcustomfields.com/resources/acf-validate_value/
