RicodJour wrote:

The non-cambium layers of a tree's wood is dead, but not dead in
the way animals die. Wood is pretty stupid, pardon my French -
it doesn't _know_ it's dead. So it keeps on trying to fulfill
its function which is to transport water.

Again you are talking about the recently-dead layers (xylem) immediately
under the vascular cambium. Under that layer is the secondary xylem,
which no longer conducts water and is used to store waste products (in
some cases - resins). The secondary xylem is also known as heartwood,
and this is what is used to produce modern dimensional lumber.

Both the Xylem and heartwood (secondary xylem) are composed of dead
cells. They are structurally intact, but they no longer respire (ie -
they are not biochemically active).

When a limb of sufficient size is cut near the trunk, you will be
exposing this dead heartwood region, which is incapable of protecting
itself against weather, sun, fungal and insect dammage. Only new
cambium growth over the exposed area will eventually provide this
protection.

It will do this whether or not it is still in tree form or
built into a deck or whatever.

Not really. Heartwood is not a good conductor of water. In fact, it is
necessary that the heartwood not contain significant amounts of water,
since that would dammage it during freeze-thaw (expansion-contraction)
cycles in the winter.

Any coating put on a tree that is intended to protect it from water
will interfere with the tree's eons-long evolution of its healing
process.

Heartwood is not normally exposed, so the application of a coating to it
will shield it against sun, rain, humidity, fungal and insect attack.
Normally the cambium and bark performs this function.

It is important how, when and where to prune limbs. Improper
pruning will not allow the tree to heal.

Yes, there is a correct cut-line to use for the most optimal removal of
a limb from the trunk. But we digress.