Explanation
To ensure that a custom field is available to display on a given page, the Developer needs to override the Service Class that the page uses and update the Service Management in CCAdmin for the given storefront to use this new Service Class. The Service Class is responsible for retrieving and setting data for a given page.
The Logic Class is responsible for implementing business logic and validation rules for a given page. Creating a new Service Class or Logic Class is not necessary, as overriding an existing one can achieve the same result.
Salesforce References: B2B Commerce and D2C Commerce Developer Guide, Service Classes, Logic Classes