On Wed, 21 Dec 2016 13:37:16 -0500, Frank advised:

Belts are on the part of the tire that touches the road. Keeps the
tread rigid so it does not flex much on the road and wear faster. Side
wall are made more flexible and are not as easy to patch.

Thanks Frank for explaining as I didn't realize until I just looked it up
that there are circumferential steel "belts" and then there are these
"radial" body plies.
http://www.idmsvcs.com/2vmod/suspens...irecutaway.gif

If I look at these cutaway diagrams, it seems we can patch to the edge of
the steel belts, but not after that edge (where there is only the "body
plies". Is that right?
https://www.treaddepot.com/assets/im...truction-1.png