Supra (Latin for "above") is an academic and legal citation signal used when a writer desires to refer a reader to an earlier-cited authority. For example, an author wanting to refer to a source in his or her third footnote would cite: See supra note 3. Or for text in that note: See supra text accompanying note 3.

Based on this passage, when do you use supra in legal writing? Let's give stream of consciousness first:
The relevant sentence in the passage is: Supra (Latin for "above") is an academic and legal citation signal used when a writer desires to refer a reader to an earlier-cited authority.
So, the final answer is when a writer desires to refer a reader to an earlier - cited authority.