That's the common notation for natural harmonics, although the sounding note with the little circle above it also can be used in some cases for artificial harmonics (alongside the diamond noteheads) to indicate the sounding pitch in more unusual cases (harmonics that require touching two positions, harmonic glissandi, harmonics consisting of higher partials etc.), although often for these cases one just uses a smaller notehead, or a notehead in brackets.
For natural harmonics, just writing the sounding pitch like this, with the circle, is usually sufficient, as long as there is only one string on which it can be played. As soon as there are several possibilities (say, a D5 on a violin, which can be both a fifth harmonic on the G-string or an octave harmonic on the D-string), one should also indicate the string above said note (either with numbers I-IV or letters).
P.S. I don't actually know, but I could well imagine that the circle above natural harmonic notes is originally the same sign as the 0 designating notes to be played on an empty string, since natural harmonics are also played on empty strings, so to speak. Considering that, it would really make sense only using it for natural harmonics and not artificial ones. But as I said, that's just a random guess.