The design level is printed on the equipment nameplate already 22kA, 42kA, 65kA, etc.
This is from the manufacturer and is usually tempered by a phrase like "or not to exceed the value of the lowest installed device"
The field marking implies that it is information not available to the manufacturer, so it would be the calculated value.
The field marking is required to give the inspector, or anyone, a value to be used to compare to the equipment label. It makes no difference if this is a design level, an infinite bus value, or an actual calculation, without some number a comparison cannot be made.
Maybe we need to clarify some terms, like Design level:
Consultants - often put a design level on their drawings based solely on voltage levels, like 480V shall be rated 65kAIC.
POCOs - usually have design level fault currents that are meant to never be exceeded regardless of their systems variables, these often exceed infinite bus values.
Manufacturers - these are often tested values, although in the case of control panels, article 409 and UL508, they may not be.
In any case, I agree with your premise that the field value has a high potential to be misused.