9.4Example Salesforce System API

Backend:

Salesforce Account

with fields:

Id
External_Customer_ID__c
Name
BillingState
Customer_Status__c

System API might expose:

GET /customers/{customerId}

Response:

{
  "customerId": "C001",
  "name": "Acme",
  "state": "FL",
  "status": "ACTIVE"
}

Internally it runs:

SELECT
    Id,
    Name,
    BillingState,
    Customer_Status__c
FROM Account
WHERE External_Customer_ID__c = :customerId

The caller doesn't care.

That's abstraction.