$CFG->defaultcourseroleid defines a site default for the role given to

$CFG->defaultcourseroleid defines a site default for the role given to
people who enrol in the course.

$course->defaultrole defines the value for each course for what role
should be used in the same case.   Naturally it defaults to the site config
(when it is zero).

This role is retained even when the person editing the course settings
doesn't actually have that role in their list of assignable rights (however
they are prevented from actually switching to any such role themselves).

Note that enrol plugins are free to ignore these values and use whatever
roles they like.  Generally, though, they should respect these settings.