But a topic is picked by the thread not the forum, a GD has a topic about what ever besides programing
I can see where quzah is coming from: the theme of this entire message board is computer programming, especially in C and C++. Hence, a forum that is about things other than programming is "off topic". The Tech Board is another such forum, though it sometimes includes programming topics that do not fall under any existing forum specific to the programming language.
off topic is also someitmes called "cheep poist"
From another point of view, a "off topic" forum is not very useful because the only topics that may be posted there must be off topic, but once the topic is posted, it is no longer off topic and hence must be moved.