what defined as flat chested?

A chest that is flat.

Anyway someone explain to me cup sizes and lettering. Coz theres like C cup but theres also DD size. No idea how the tit scale works.

DD is just the next size up from a C, there's AAA, AA, A, B, C, D, DD, E...

a bra size is a number and a letter e.g. 32B

teh number refers to the size of someone's chest right under their boobs (so narrow skinny girls have a smaller measurement) whereas the letter refers to the difference between the circumference under their boobs and the circumference at the fullest part of the breast

so a 28D will look very different from a 38D

like idk if am flat chested or not, sometimes i love my breast size and sometimes i think theyre tiny. i feel that my breasts to bigger and smaller during the month like when am PMSing they go really sore and stuff and feel and look bigger but when ive just finished my period they go back small. so my question is, what defines a flat chest. i wanna know if im in the safe zone.

yeah am 28DD but dont even know if thats flat or not, i have a really narrow waist bc am slim and even 28 back is 'a bit loose on me'