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Bottom Bracket Model

Bottom bracket model tells you the Crank Type:

 - BB-UN100 square taper cartridge
 - BB UN26 Square taper
 - BB-MT500-PA Hollowtech II
 - SM-BB52 Hollowtech II
 - RS500 Hollowtech II Sora/ Gravel bike
 - ES300 Octalink

 - If you are trying to replace a standard square taper BB, you must specify the shell width and spindle length. Eg: 68mm and 127. The Shimano BB-UN55 is very common and comes in 3 widths and 7 spindle lengths.
 - if you are trying to replace Hollowtech you don't need to know anything. The hollow spindle is part of the crank, not the BB

 - if you are replacing the whole crank system with another, you need to ensure the existing chainline is maintained. Eg: If you replace a UN-55 with HollowTech, you need some idea of what the chainline will be.