Stage is a communication protocol processing stage. It denotes all processing that is initiated in response to a request. Stage is named by a type of request that is processed. Some macros, engines and pipelines may be not applicable at some stages. This should be taken into account when designing processing logic.
Smtp-end-of-data stage is initiated after MPP receives end-of-data sequence from its SMTP/LMTP client. It is the final stage of SMTP protocol. At this stage everyting about email message is available for processing: SMTP envelope sender and all recipients, email headers and body, etc.
Policy-rcpt stage is initiated after MPP receives Postfix Policy server's request with state=RCPT. At this stage client IP, SMTP envelop sender and one recipient that is currently received are available. No email headers or body or anything related to email data is available.