What is the difference between Customer ID and Username?

Customer ID is a unique identifier for a Quik! customer account (a firm or organization) that cannot be changed. It is always a number that begins with 1001.

Username is an identifier for every user within a customer's account. By default, when customer accounts are created, the Username for the master user (i.e. first user) is set to the same value as the Customer ID. Usernames can be changed, and can be alphanumeric.