I think there are some misunderstanding about Post, reply and topic. Let me clarify it. And if I miss something, please adding more details information to correct them.
We should distinguish differences between those three terms in the forum.
1) Topic:
A topic is a discrete piece of content that is about a specific subject, has an identifiable purpose, and can stand alone. For example: Your topic, namely "posts and replys", it should be replies, not replys (typo)
https://en.wikipedia.org/wiki/Topic2) Post: A post, also called a thread, is something like your comment on Facebook, Twitter, etc. in order to show/ illustrate your ideas. A post can discuss about the topic's question, it's not related to anyone else post in the same topic. It's simply your ideas.
3) Reply: A reply is a post, which is posted for specific purposes, such as answering or discussing ideas of someone else.
In the forum, you can reply by using Quote button in someone's thread, then typing your ideas. That is a reply.
For example:
This post is a post/ thread
The second one in the following link is a reply to topic creator's OP:
https://bitcointalk.org/index.php?topic=2871735.msg29757069#msg29757069