I think being discreet is all about being accepted. About not unsettling anybody. About avoiding unwanted attention. About feeling safe.
In my opinion, most of us try to exercise discretion. And, depending on the environment and circumstances, we may try to be especially discreet.
I also think, discretion is about awareness and experience. And the ability to be discreet.
When I was young and developing breasts, I was oblivious or in denial, so, I was not at all discreet. But, from all the comments, and unwanted touching, and forced display of my breasts, such as in PE, I learned about the importance of modesty. And, with my mother’s help, how best to camouflage. She also helped me learn when it was safe to discreetly wear a bra and how and when not to.
I have carried forth those lessons about modesty and camouflage and discretion into my adult life. However, I have learned that the calculus changes with the size of your breasts. When I was a mere B cup, camouflage and modesty were simpler, and although it was more comfortable to wear a bra, I could easily go braless when discretion dictated.
Now that I am a D, seemingly quickly closing in on DD, camouflage is impossible and modesty dictates pretty much always wearing a bra whenever I go out. When I am braless, all the movement on my chest is both suggestive and physically uncomfortable. Also, it’s quite a provocative look when the shape and size of my breasts is readily apparent and my nipples showing through my tops. So, discretion is no longer about whether to wear a bra. In fact, discretion now requires that I do wear a bra. And discretion purely about what top I wear with my bra.