Top-posters reply to a message above the original text; bottom-posters, the opposite. Interleavers reply within the original text; while copiers pick out what they want to reply to and copy it above their response. Law departments may become accustomed to one style or another, usually one that is in accord with the style of the senior lawyer.
The preceding distinctions came from the Fin. Times, Oct. 4, 2006 at 8, which also offers a rule of thumb. “If you are beginning your third or fourth meaty paragraph, consider a white paper … or at least a phone call.”
My personal peeve is with excessive reliance on e-mail, especially long ones that require me to scavenger hunt for the key point or question. Forwarders should delete the extraneous stuff from long chains of messages. Far better: pick up the phone and call!