When a normal task chair is not enough
Posture chairs built around back support rather than general comfort, including the Chiro Plus and Chiro Plus Ultimate high backs. These are what to specify when someone is having genuine trouble, not just when a chair feels old.
What makes them different
More pronounced lumbar shaping than a standard task chair, and usually independent adjustment of the back angle and seat angle so the two can be set separately. That combination is what lets a chair be fitted to a person rather than approximately fitting everyone.
The Chiro range is the usual recommendation where someone is at a desk all day and reporting lower back pain.
The honest limitation
No chair fixes sitting still for eight hours. A posture chair makes prolonged sitting more tolerable; it does not make it good for you.
If back trouble is the driver, a sit-stand desk alongside a decent chair usually does more than an expensive chair alone, because it changes the posture rather than supporting one posture better.
Fit matters more here than anywhere
These chairs are adjustable precisely because the support needs to land in the right place on the individual spine. A posture chair set up wrong is no better than a standard task chair, so allow time to set it up with the person who will use it.