This article covers 

How do we differentiate between billing email and billing contact email in the email configuration?

I want emails to be sent to the customer’s email and the billing contact email only. How do I accomplish this?

Summary

Apart from the customer's email address, there are other email contacts that can be added for specific email communication. The difference between the billing email and the billing contact email is as follows: 

Billing Email: The email address that is part of the customer's billing address. 
Billing Contact: The additional contacts that are added by checking the “Billing Contact” checkbox are called billing contacts. This email address will receive invoices and payment-related invoices.

Solution 

Apart from customer email (the primary email address for the account), there are four kinds of email that can be added. 

a) Account contact
b) Billing contact

c) Billing email

d) Account email
e) Shipping email

The billing contact is the email address that is added as a part of the customer’s billing address.
Below is a screenshot for your reference.

This is different from the customer’s email address.

All email notifications from Chargebee on the occurrence of the respective event are sent to the customer’s email address.

Now Chargebee also provides you with a feature to add additional email addresses to a customer called “Additional Contacts” and this option is available under the Customer Details page.

You can add them by clicking on the Add additional contacts option on the customer details page. The additional contacts that are added by checking the “Billing Contact” checkbox are called billing contacts. This email address will receive invoices and payment-related invoices. By checking the "Account Contact" checkbox, this email address will receive subscription related invoices. Refer to this link for more information.

Shipping email

Apart from other email contacts, the shipping email will be available only under the Subscription Details page, and this contact is used for products shippable. 

Email Notifications

Now if you are looking to send email notifications, let's say invoice receipt email notifications, to the customer's email and the billing contact's email only, you need to navigate to Settings > Configure Chargebee> Email notifications> Invoice receipts, click on the “Edit” option, enable the checkbox for “Send to Billing Contacts,” and publish the changes.

Note: 
1. The first email notification sent during signup or when creating a new customer can't be sent to the additional contact because an additional contact can only be added to an existing customer. 
2. You can add up to six additional contacts for a customer. 
3. You will need to enable the checkboxes as per your requirements to each of the email notifications manually on your Chargebee site

Related articles and Documentation

Additional Contacts
Configuring Email notifications