[EricH]

You raise a good question.

The TRAILER is a field element in and of itself (6.4). Robots may not attach to any field element (exception: robot towing trailer).

My interpretation: The exception is for the robot that is currently towing that trailer through the hitch. That robot alone can attach to the trailer (through the hitch), and no other robot may. Robots doing so are subject to the penalties in the Manual.

I would also check with Q&A just to be sure, but don't plan on attaching to opponents' trailers.