This page deals with slang terms for a Zombie apocalypse, in the heat of the moment it's easy to garble in horror at the situation and important messages are lost so easy terms are introduced by the military which are inevitably adopted and mutated by civilians.
In art, an underpainting is an initial layer of paint applied to a ground, which serves as a base for subsequent layers of paint. Underpaintings are often monochromatic and help to define colour values for later painting.
Basically, sinus tracts & tunneling is the same thing, just different terms. Undermining is a wider area of wounding that lies beneath the wound opening. Think of undermining like this: the wound is larger than the hole or entrance of the wound. Tunneling is like a cave, undermining is like a cavern or cavity.