It has to be restricted access to only qualified personnel, in order to take credit for the setting you select. Once it qualifies, all calculations that depend on the adjusted setting, are permitted to take credit for it. Ungrounded sizing, neutral sizing, EGC sizing, selective coordination of time-current curves, etc. Otherwise the highest possible rating applies.
240.6(C) spells out ways you could do this that qualify. IMO, qualifying with part C without even thinking about it, is the most common situation where you'll use an adjustable trip setting in the first place, because it would likely either be behind a locked electrical room door, or behind a lockable equipment door, or both. Any breaker in a panelboard intended for the general occupants of the building to use (like the ~200A panels in a dwelling unit), will likely be a fixed trip breaker.
The intent is the concern that the general public might change the setting unaware of the consequences, it would make the circuit unsafe, if not sized to worst-case-scenario.
