Question

what does J stand for in the course 6.207j

Answer 1

In general, the letters are a focus:
http://mitadmissions.org/blogs/entry/numbers_are_names_too

By looking at the Course 6 PDF here:
http://web.mit.edu/catalog/subjects.html

I see all the J ones are things like "Cellular Biophysics and Neurophysiology," "Quantitative Systems Physiology," and "Fields, Forces and Flows in Biological Systems." So I would say bio related sub topics.

Answer 2

I believe that the J suffix indicates that it's a joint course with another department (effectively a codeshare in airline terms).

Answer 3

I thought the J courses were originally thaught in January as enrichment or extra curriculum courses