I grepped this from GTAC:
A "discipline" in SM has to do with a placeholder of sorts; a container where people with common ground might exist. You might have a Discipline called "Tech Writers" where every Tdoc person gets added to the Discipline. A "role" has slightly different meaning in TC core as opposed to SM. In SM, you can be a Coodinator, a Participant, or an Observer. In TC core, you can be a dba, a Project Administrator, a Resource Graph Viewer, etc. Roles have to do with ACCESS AND CAPABILITIES. Disciplines have more in common with GROUPS then they do with ROLES.
SM = Schedule Manager
Maybe helps to understand the difference.