What do you class a non harmony note as when it plays over harmonic accompaniment

I’m now trying to transition from 4 part harmony (with non harmonic passing notes inbetween chords etc.) to different textures: melody and harmonic accompaniment etc. rather than just stacked chord followed by stacked chord etc. My question is, if you have a whole note stacked C (CEG) triad (for example) underneath a melody and the melody run includes B and D notes, what are the rules for being able to use those notes on the chord? In 4 part harmony adding these notes ontop would make the triad in fact a seventh or ninth chord. So in doing the same but with melody and harmonic accompaniment rather than just 4 part writing, does it just mean with the triad playing underneath, you’re just implying the momentary sound of a seventh chord (in the case of the B note) or a ninth (in the case of the D) when using them in the melody? Thank you to whomever replies.