For synchronous VSAM RLS requests only, by specifying RPLWAITX the application can provide an RLSWAIT exit that issues WAIT for the completion of the request. When the request is completed, VSAM POSTs the request. The RLSWAIT exit ensures the ECB is marked POSTed before returning to VSAM. Note that RLSWAIT is entered only for a request completion wait, never for a resource or I/O completion wait. See z/OS DFSMS Using Data Sets for more details.