Quick answer
Internal clearance describes the relative movement between the bearing rings before mounting or under a stated measurement condition. A greater-clearance designation such as C3 does not mean a higher quality grade. Preload is a different design condition that creates an initial internal load.
Check the mounted condition
Interference fits and temperature differences between rings affect operating clearance. The unmounted catalog condition is not the complete application requirement.
Do not substitute an arbitrary suffix
Normal, C3 and other clearance groups must match the relevant bearing type and manufacturer definition. A base model or matching outside dimensions do not establish the correct clearance.
Describe the operating conditions
Include shaft and housing fits, speed, temperature, load and required stiffness. Angular contact pairs may need a specified preload and matching arrangement rather than a radial-clearance choice.