The low back is defined by the lumbar spine, composed of 5 vertebrae, named L1-L5. The pelvis is defined by the bones of the pelvic girdle.
There is an old saying that no posture is bad unless you get stuck in it. The problem is that the thoracic spine often does get stuck in bad posture.
Two opposing muscle groups, the “facilitated” muscles that are locked short and the “inhibited” muscles that are locked long.