UnsupportedProgram

One construct the configured interpreter cannot evaluate — a stored rule, an implicit rule, a computation, a field declaration whose custom type is unregistered, or a single document cell.

[subject] says which of those [path] addresses, and it is load-bearing rather than descriptive: an [UnsupportedProgramSubject.ELEMENT] path is a model address a consumer can look up in the model, while a [UnsupportedProgramSubject.CELL] path is an instance pointer into the document being evaluated. Both kinds arrive in the same list — a support audit reports only elements, but validation and computation report both — and before this field a consumer had no way to tell them apart, so it could render neither correctly.