Hi Gopal,

There's some info on Role contexts here: 

ttps://help.totaralearning.com/display/TL12/Roles#Roles-Rolecontexts

A user can be enrolled into a course as a 'Learner' for example but assigned the role of 'Trainer' in a Seminar activity.

Basically Enrolment gives access to a course, but assigned roles determine what capabilities the user has within certain parts of the course.

Hope this isuseful,

Cheers,

George.