HTTP API sub-users allows you to provide an HTTP API access to your customers or to setup different limits and access to the parts of your system. For an example you can setup a different HTTP API sub-user for each of your web sites, servers or control panels and each sub-user will have an access only to its resources and will not be able to access the data of the other sub-users.
You can navigate from the left menu to see how you can create and modify sub-users to directly with a main HTTP API user.