How do you describe the relation between a course instructor to his teaching assistant? Is he the "teaching supervisor" to his TA? This is encountered during PhD application. Thanks!
It sounds like the question is that you need to fill in the "In what context have you known the applicant" box on a PhD recommendation letter.
If this is the case, then, for this particular point, I would not list "research supervisor," since this is clearly a teaching situation. However, "student" also doesn't make any sense. So, if there's no specific option which clearly satisfies this, I would check off the "other" box, fill in "teaching supervisor," and then explain the relationship clearly in the text of the recommendation letter itself.