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Is my understanding correct for this Shimano BB UN300?

The shell width is the green length below and the axle length is the red length below. So when I buy this BB I need to tell the dealer both lengths?

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Yes, you need to specify both parameters. In this case 68mm is the shell width and 127mm the axle length. The diameter and thread type must match, but they are often the same BSA standard. The letters D-EL/D-NL and likewise refer to a specific intended chain line if I am not mistaken. It should ideally match as well but this is not as critical as the other parameters.

Your figure is wrong. Shell width is not equal to the width of the bottom bracket body itself shown in green. The body is slightly wider than it's intended shell as it has a lip that sits on the outside of the shell.

You can see this effect in the picture i posted in this thread: Bottom bracket BSA, 68mm vs. 73mm

EDIT: Just to make it clear: If you have a 68mm shell on your bike, you should order a 68mm bracket (which will be slightly longer than 68mm if you measured the green length).

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