[compiler-guy]

The C and C++ standards include "Implementation defined behavior", which means that a conforming implementation can do whatever it wants, as long as it specifically documents and sticks to that behavior.

This doesn't really help portability all that much.

[Karliss]

That's a different category. Standard defines and uses all 3 "undefined", "implementation defined" and "unspecified" behavior. The difference between last two is that compiler isn't required to document exact behavior. Unlike UB triggering it doesn't automatically summon nasal demons and range of possible behaviors is usually described by standard.