What is the difference between class A, B and C surfaces in surface design?

The distance between each point of the edges of two neighboring patches must comply with the following limits:
• For class A: no more than 0.01 mm.
• For class B: no more than 0.02 mm.
• For class C: no more than 0.05 mm.
Tangents continuity
The angle between the tangents to the surface on the edges of two neighboring patches must comply with the following limits:
• For class A: no more than 6’ (0.1◦).
• For class B: no more than 12’ (0.2◦).
• For class C: no more than 30’ (0.5◦).
Curvature continuity
The control parameter is the patch curvature along its contour.
• Class A surfaces must have coincident curvature at least every 100 mm of
contour of two neighboring patches.
• Class B and C have no applicable rule.
Points of maximum curvature or inflection are only allowed along patch contours of Class A surfaces.
Completion surfaces
As we have seen, they do not belong to the aesthetic surface (even if part of them is visible and influences the aesthetic evaluation of the body), but are added to represent the part as it can be produced.
• For class C surfaces: completion surfaces are not required; gaps are represented by double lines drafted onto the aesthetic surface.
• For class B surfaces: only rounded contours are represented on the surface
that is always fixed to the body.
• For class A surfaces: completion surfaces must be designed in all detail.
Shape tolerance
The shape tolerances are measured by comparison of a surface patch with the
counterpart on the aesthetic surface.
• For class A surfaces: no more than ±0.5 mm on body shell surfaces (or large
surfaces) and no more than ±0.2 mmfor interior trimming (or small surfaces).
• For class B surfaces: no more than ±1.0 mm on body shell surfaces (or large
surfaces) and no more than ±0.5 mm for interior trimming (or small surfaces).
• For class C surfaces: shape tolerance is not applied.

Btw i copied this answer form the user named : Zoran Andjelic.
So all credits to him

Thanks steendelaar..

A surface means visible surface.it will give styler.B surface means thickness of a part it is not visible it is inside part. c surface means A and B combined to give C surface