A topic title for any thread is set by the title first given to it. 'Subject' is blank so you fill it in.
However, no one seems to edit it as the thread goes on, although to do so can sometimes be useful.
I have a long standing thread, starting with the ships name. I often change the end of the title to reflect something new I am doing in the build, whilst always keeping the beginning part for continuity.
Am I the only one?