Classic harmonies are useful for generating options. They do not prove that a palette works, so use this to start and judge the result in use.
One colour against the two beside its opposite. Softer than a complementary pair.
2 colours were reduced in chroma to fit a standard screen.
This is a structured starting point. A set of hue angles says nothing about proportion, lightness or context, and those decide most of how a palette looks.
Use it when you have one colour and need candidates for a second and third, including for outfits and rooms, where it works as a prompt and not a rule.
No hue relationship is correct in itself. These formulas are design conventions, not tests of what any person will like. Proportion, lightness, chroma and setting matter more. Complementary pairs of equal strength can vibrate, for example, and a muted version of the same pair can be calm.
Hues are placed around the OKLCH circle, which keeps lightness steadier between hues than the HSL wheel behind most harmony tools. Chroma and lightness start from your colour and can be moved together. A colour that cannot be shown on a standard screen is reduced in chroma, and the tool says so.