Just a few points I didn't notice from others.

T-111 stands for Texture 111 which is what it was called when
it was first made.

It is 5/8" exterior grade plywood which is commonly made out
of southern yellow pine (less expensive) or Douglas fir. It has
vertical grooves spaced 8" (I think) and the ends are lapped.

It was originally designed to replace the sheathing on a
house. The idea was that it could be nicely stained and would look
decent for a couple of years until the owner put real siding over it.

In our part of the country (northern NH), it was introduced in
the early 70's. Some folks have left it exposed for 30 years and so
long as it's kept painted or stained it seems to hold up ok.

FWIW,
--

Larry
Email to rapp at lmr dot com