What is the bar at the top of the HTC 10?

I read somewhere that the plastic area you are describing is for the various antennas near the top of the phone.
 
Yep, from the latest AC review:

"Up top, that extravagant metal design is broken up by two engineering necessities — a 3.5mm headphone jack, and a plastic section for antenna reception. It's easy to ignore on the dark gray model, but stands out a little more in the gold and silver versions."