Technical, I agree with guivho. Many forums indent or number replys so they are directly attritbutable to the post to which they are responding. e.g.:

Question
Reply1
Reply2
Reply to Reply2
Reply3
Reply to Reply3

Since some replys are to replys rather than the original question, using only sequential numbering of replys is somewhat confusing unless the reply includes a quote from the message to which it is replying.