Non-Canon means a couple that either never gets together or doesn't get together without some kind of outside interference - Saavik has sex with teenage Spock's mindless body, but that doesn't really count as a canon pairing. Chapel/Spock is also a non-canon pairing because they never get together (except in that one cartoon episode with Harry Mudd's love potion), although Christine sure would have liked to!
If two characters get together but then don't stay together for some reason (Trip and T'Pol) it's still Canon.
I would classify Doctor/Seven as non-canon because it's a one-sided deal in the seris - Seven doesn't express interest in the Doctor, although he loves her. T'pol and Reed would be non-canon because even if he liked her, they never willingly kissed or dated or slept together.