The Internal API requires the fleet to give you an API key. This will enable you access to far more tools and advance features unlike the Customer API. For example, an Internal API can manipulate job pricing, where-as the customer API is not able to.
The Customer API is given to every single customer without restriction or approval required (unless their account is suspended or deleted by the fleet operator). There is no need for the fleet permission, and the API can be accessed by using the very same logins used in the mobile application or login panels, a username (generally mobile number) and password (generally a 4 to 12 digit number). The amount of tools available via this API is limited, and the ability to obtain certain data or change data is heavily restricted.