The voice column index is used to determine the positions of notes when multiple columns are needed; for example, when notes in two voices are a second interval apart and therefore cannot be placed directly above each other vertically, instead they must partially overlap.
Dorico Pro automatically changes the voice column of voices according to the number of active voices and the pitch of notes. Dorico Pro prefers showing voices with the widest pitch range between them on the left of the rhythmic position and voices with narrower pitch ranges to the right, as this produces the most balanced result, especially when there are multiple accidentals.