There's no 'context' for plugins. You can allow roles to be added in a number of places (e.g. site, course category, courses and so on) but 'plugin' isn't one of them. Where you want to do this probably depends on what sort of plugin we are talking about.
However, Manager roles are generally at a higher level - site and course category. They don't make much sense elsewhere.
However, Manager roles are generally at a higher level - site and course category. They don't make much sense elsewhere.