Do you remember how it looked like?Quite some time ago there was a function in here that let you "fork" a thread - in effect start a new thread as a split-off from an existing one, with links etc. built in.
I seem to recall there was a "Fork Thread" button somewhere - maybe next to the "Reply to Thread" button below the last post? Clicking on this launched you into starting a new thread and put the links in for you, and (if memory serves) put a note in the original thread showing it had been forked, along with a link to the new thread.Do you remember how it looked like?
How did it work, as it were. (Not on the server side, obviously, but how did users interact with the fork and what was different from merely starting a new thread?)

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.