Question
Execution of DomainNamesAvailability_API returns PlanID Int32(NULL) in the response, according to the documentation Plan ID is to be returned
Answer
This is a documentation issue. The API method always returs PlainID Int32(NULL) in case the domain is not available for registration.
A request #PBA-39892 ("Incorrect description DomainNamesAvailability_API return parameters in case domain is not available") exists to correct the documentation accordingly.