reviewAction extension for adjudication slice is used for capturing the review details (decision & decision reason) for prior auth. 

We have this extension element available at item level (product/service) in HL7.FHIR.US.DAVINCI-PDEX\PDex Prior Authorization - FHIR v4.0.1

However if we want to capture review details at Authorization level, we would need this element at header level adjudication slice [ExplanationOfBenefit.adjudication]. But the adjudication slice at header level doesn't have reviewAction extension.