Bicycle tire and wheel sizing nomenclature is unbelievably arcane and confusing.
https://www.sheldonbrown.com/tire-sizing.html
Basically it stems from a history of different countries standardizing different naming schemes that corresponded to different measured characteristics of the rolling stock on a bicycle. This has not been improved with more recent invented nomenclature like the 29'er. What pisses me off the most is that the letters following the numbers of designations like 650b and 700c, were originally meant to denote the width of the tire, with A being the skinniest and D being the widest. Now these letters have absolutely no relation to the actual width of a tire which bears the letter.
Bead seat diameter is the most important practical consideration when looking for tires that fit your existing rims these days.