It is one-off because the operation is very specific to a particular purpose. It cannot be used for other conversions and if either FHIR or X12 evolves, the operation would need to be revised. As such, the approach is more fragile than simple REST interfaces which are use-case independent. Will change the wording "'Low' indicates that the solution is one-off and support for additional use-cases using the same mechanism is almost certain to require additional development." to  'Low' indicates that the solution is generally use-case specific and support for additional use-cases using the same mechanism is almost certain to require additional development."