"Off topic" means not according to the suject in the title of the forum topic.
"On forum" means worth posting in public.
"Off forum" means not best posting in private to which ever individual the comment was intended.
These are general terms applying to discussion forums and mailing lists in use across the internet widely. A good forum requires a little discipline on behalf of the contributors. One of the problems of forums is that conversation tends to drift, or individuals wish to chat between themselves, making it difficult to follow the original thread.
Some individuals, especialy grumpy old-timers, will often complain that posts are "off topic" and discussion should be moved "off forum".
It is possible to copy and paste posts and take them else where, to other topic that is more suitable, or to start a new topics if they are that interesting. To do so;
- Click on the quote link of the post you wish to reply to
Copy the contents of the original post
Go to the forum or topic thread that you wish to write in
This time click on the reply, not quote button
Paste what you copied into the Post a reply window
Edit it down to the respective part you wish to reply to.