sub frame = attaching to bottom rear end of tractor (possibly some place in and around the final drive units), most likely some place on transmission, and then ran up to and connected to the framework of the FEL.
there is more likely a couple holes on backhoe. for were it would accept a sub frame holes/pins.
your weak spots without sub frame, is going to be 3pt hitch lift arms and top link, then the pins that connect them to tractor or rather metal casting of the rear end, and then all the bolts between rear of tractor to front frame that the FEL attaches to. when operating a backhoe. and say lifting up rear end of tractor off ground, to compacting dirt with backhoe bucket, to other..., you end up beating up the weak points. a sub frame is there to transfer the beating from weak points to the sub frame. a backhoe is just not a typical regular 3pt hitch attachment/implement that just gets dragged behind tractor.
as much abuse / damage i have done over the years. i might say you could un-intentionally split your tractor in half (as if you were going to service the transmission) but in this case shearing the bolts in half from use with backhoe without a sub frame. granted 3pt hitch linkages and like would more likely fail first. but... hey it could happen. this is more of an attempt to get you to look at and possibly understand why a sub frame may be important.