Neither of those.
A “Topic” is a conversation, a volley or round of ideas, each flowing from what came before. As we exchange thoughts, we are affected both consciously and unconsciously by all of the individual comments, even if we are not directly replying to them. Our understanding shifts, we have new information or viewpoints to integrate.
It is fundamentally false to think of a string of direct replies as independent of the rest of the discussion.
Caveat
If a set of replies is truly so divorced from the flow of the rest of the discussion that it benefits from being viewed in isolation, it’s an excellent candidate for being broken out into its own top-level topic.
This is why temporal ordering is important for both reading and composition. For a specific example, our own back-and-forth here would end up as a tree of replies on a platform such as reddit, would it not? However, they don’t really stand alone.
- If you were to read only that reply string, without the context of intervening posts, It really does sound nonsensical and disjointed - as well as possibly coming across as more acrimonious than I intended.
- If I had to compose using only the previous direct replies, we probably would have stopped engaging after one or two comments, or at worst, started yelling at each other.
Appropriately enough, it was this exact process - watching the discussion as it unfolded, including the parts that I didn’t directly respond to, and thinking about why I act as I do - that has allowed me to articulate and refine on what had previously been a strong visceral reaction. A threaded model would have deprived me of this experience.