Some game objects such as trees, bushes, and fences do not need and therefore do not use a class. Other objects such as houses, things with ladders or doors, etc require a class. Only classed objects can appear in the mission editor, although the default BI mission editor config does include all classed objects.
To the best of my knowledge, the 6th sense editor upgrade only adds classed objects to the editor. This means some objects still can not be added into missions, but it allows users without any editor update (EU) to play a mission created using it.
Other EUs define and add new classes to the editor, but will require the user to use the same (or a compatible) EU just to play the mission. Try the EU by LowFly, although i think the link is down. It may be unwise to create a mission that requires a specific EU unless it adds significantly to the mission. At the moment there is no standard EU as was the case in the latter years of OFP's life.
Mozi - I do not believe it exists. I hunted long and hard for the 6 foot fence with the barbwire also. Using the 6th sense editor to find it won't help - been there, done that.
Nuxil: The fence you speak of us barbed wire only, and sits about 3 feet tall with wooden posts at it's ends. Different.
Why does this matter? The barbed wire fence Nuxil speaks of can't be run over with trucks and such - whereas the nice barbed wire fence Mozi seeks, the one commonly found all over the Sara map itself, can.