For parsing the inbound JSON request by the Service-REST, you need to use Map To as JSON to convert JSON request and map the elements to a page or pagelist in the clipboard. The Pega data model to which you are mapping is replica of the JSON content (key value pairs). You don't need to map the keys and values explicitly. The JSON content is converted and mapped to the clipboard structure per your data model in this case.
But my question is not about how JSON mapping works. It is about, why the labeling of the fields (Map to, Map From) on the request/response tab is such way? It is confusing me as why Pega uses Map to and Map from labels interchangeably although I know what to do to make it work by referring Pega Help documentation.