Apparently, order is driven by 1) the date / time of the most recent "New Topic" post, 2) the most recent response to any of the New Topic posts. Whichever event happened most recently will place that "Topic" at the top of its respective Index. At the old MarketForum site the position of the post was purely determined by the date and time of the originating "New Topic" post (FIFO).
I determined this by responding to the topic "Why isn't our username showing up with our posts?" At the time I started posting my response, the Topic was listed number 21 of 21 (last in its Index). Once I posted the response, the listed order of the Topic in its index had become number 1 of 21 (first in its Index).
This suggests that a hot-button topic that was beaten to death months ago could suddenly be brought to the top of its Index because someone decided to respond to it way, way after its origin.
Somehow, I am not looking forward to that happening.
I see pluses and minuses to that. We're very used to the fixed positioning of topics where the relative order is constant. That desn't work in the current setup and I'm finding that it takes a little extra effort to find specific posts. I suspect that in time, we'll retrain ourselves.
I find it chaotic; I liked knowing that if a new topic was introduced two days ago it would be further "down" in the index than one introduced yesterday.
There's already too dadgum much change in the world as it is without the damn topics flip-flopping around all over the dadgum board, and dadgummit stay off my lawn!
One of the plusses to that, is that you don't have to scroll through a bunch of posts to get to see the response to that particular post, made a few days ago. Kinda like facebook or twitter.
OTOH. You can scroll right past it if it doesn't interest you.