All users on Tableau Server must be assigned a license level, there are three types of license levels: Unlicensed, Viewer, and Interactor.
You can have more details from the following link.
Is this only for license?
I thought of it as a web service interactor service from tableau server which I never heard of. I guess thats why I couldn't get any info about the topic over the net how silly of me
Thanks a lot Indumon
Glad it cleared your confusion!