Clearance before mounting
CN is normal clearance, C3 is larger than normal and C4 is larger again. The selected group refers to a controlled measurement condition, not necessarily the residual clearance under operating temperature.
Fit and temperature reduce clearance
Interference fits, differential thermal expansion and axial clamping can reduce residual clearance. A hot shaft or aluminium housing may require a larger initial group, but this must be engineered rather than guessed.
Avoid using clearance as a noise fix
Changing to C3 can alter vibration and load distribution, but it does not correct an unsuitable fit, poor alignment or incorrect lubrication.