What is the merchantID? How does it relate to the employeeID?

The merchant ID is an ID assigned by Clover to the business. A business may have multiple devices and employees. The owner of the business (along with managers and employees) each have an employeeID associated with their account. When looking at the OAuth Response keep in mind that you can see who the current user is by using the employee_id. You can use the merchant_id to see which business the user belongs. You must use themerchant_id when making API calls.