Bra sizing is unusual in clothing because it is calculated rather than measured directly. The cup letter is the output of a subtraction, and that structure produces most of the confusion around it.
The letter is a difference, not a volume
Cup size is derived from the gap between the bust measurement and the underbust measurement. Each step in that gap corresponds to the next letter.
Because it is a difference, the same letter describes a different volume on every band. A cup on a small band holds considerably less than the same letter on a large one.
Which means a letter quoted on its own conveys almost nothing, even though it is the part of the size everyone remembers and repeats.
Sister sizes follow from the arithmetic
Moving down a band and up a letter, or up a band and down a letter, keeps the calculated volume roughly constant. These are sister sizes.
They exist because the system is arithmetic, and they are the reason a fitter may hand over a size that looks unrelated to the one the customer asked for.
Understanding the relationship converts a confusing recommendation into a predictable adjustment, which is the practical value of knowing how the letters are generated.
The band carries the load
Most of the support in a bra comes from the band around the ribcage rather than from the straps. A band that rides up has stopped doing that job.
Loose bands are extremely common because shoppers size up for comfort, and the immediate effect is comfortable while the support transfers to the shoulders.
Straps then carry weight they were not designed for, which is the source of a large share of complaints about digging and discomfort.
Systems differ between countries
Band numbering, letter sequences and whether measurements are taken over or under clothing all vary between national sizing conventions.
Some systems insert additional letters between the standard ones, so the same body maps to different labels depending on where the garment was designed.
Online shopping mixes these systems on one page, and a size converted numerically between them frequently does not correspond to the same garment shape.
Shape is not captured at all
Two people with identical measurements can need entirely different garments, because breast shape, root width and tissue distribution vary independently of volume.
Cut styles exist to address that, but no part of the size label records which shape a garment suits, so the information has to come from trying it on.
This is why fitting remains hands-on work in a category that has otherwise moved online, and why measurement alone rarely resolves a persistent fit problem.