Nevertheless, in this article on fishing reel bearings, at least relative to ball bearing tolerances, we're proceeding under the assumption the ABEC 1 (lowest) through ABEC 9 (highest) standards, which really only deal with "tolerances", are reliable indicators of "dimensional precision".
If you're into just fishing and not maintenance and repair, just focus on the number of ball bearings in a reel and the material from which they are made. A manufacturer will include this in its reel description. That's all you need to know. Keep in mind, the number of fishing reel bearings does contribute to whether a reel performs poorly or if it functions at a high level.
The "rolling action" of fishing reel bearings or ball bearings in any other device, which is what defines their quality, depends on the roundness, precision, and hardness of its components. High quality ball bearings like those found in upper end reels have extremely close tolerances as defined by ABEC standards. This translates into very little play and almost friction free rotation. Less expensive, low end reels may have bearings made of chrome plated steel, chrome steel or other inferior materials. The function and durability of these bearings falls far short of "precision" stainless steel or ceramic ball bearings.
Most reels today, particularly high grade reels whether spinning or baitcasting, use precision, stainless steel "radial ball bearings". However, more and more solid and ceramic coated bearings are making the scene. The radial ball bearing units we're talking about consist of an inner and outer ring within which is a cage, made of plastic or steel, containing a number of precision balls. The cage functions to keep the balls separated. These fishing reel bearing units are usually of deep-groove construction which accommodates both radial and axial loads common with fishing reels.