That surface changes how the string feels and how much noise it makes when you slide your fingers:
- Roundwound: textured surface, the traditional feel most players know.
- Semi-flat: sits between roundwound and flatwound in surface feel and reduces finger noise rather than removing it.
- Flatwound: smooth, polished surface with very little texture.
We wouldn't reduce the tone difference to a simple rule. Tone depends on the guitar, pickups, amp, pick attack and string age as well as the string surface. Playing technique and string age also affect how much finger noise you hear.