I apologize for the delay in answering - this post did not show up in my feed until today!
I so not see this as marginal labels. If you see it as marginal labels, then the first 'column' (Plan) is a marginal label for the 2nd column. I see this as three lists: Plan, Think, Write. I would make each of those cell 5 headings and then list the items underneath. This is telling the student to make a Plan (overall process), Think it through (shows more detail) and Write (actually do it). Making the 2nd column marginal labels adds a lot of 'stuff' for the reader to get through to find the math.