Difference between customerId and customerDomain

Question

What is the actual difference between these two fields?

The API says customerId is non-persistent, but that gives no insight as to whether customerDomain can be changed or not.

Answer 1

Both the customerId and customerDomain fields are the customer's primary domain name. There isn't any difference between them for the purposes of the API except that the customerId is always required while the customerDomain is not.

For reference, the properties are described here: https://developers.google.com/google-apps/reseller/v1/reference/customers