I think the arrow might be used when responding to a specific post in a thread. If one didn't use the respond button, leaving the original post ( if it was an excessively long post or something), the arrow may signify a response to the previous question. Does that make sense?
You know, the only other think that comes to mind is...well...Pi Phi...since our symbols are arrows and angels.
I know, ruthless pitch for Pi Phi. Haha.