I just assumed this because yesterday we did a lot of planking exercises in class and the instructor kept specifying to have your feet directly under the anchor hanging point. Of course I haven’t tried it in any other way because I don’t have my own straps at home. Good to know you are still able to do these exercises without the ceiling mount.
Couldn't find a picture or video of using a TRX on a fence, but came up with a Canadian blog entry that lists all sorts of ways to mount a TRX.
1-6: indoor options
7-10: outdoor options