set when the server responded with EmptyQueryResponse to sql query this row block represents.
Set when the server has sent PortalSuspended due to reaching nonzero result-row count limit, requested in Execute message. The appearance of this message tells the frontend that another Execute should be issued against the same portal to complete the operation. Keep in mind, that all portals are destroyed at the end of transaction, wich means that you should not carelessly send Sync message before receiving CommandComplete when you use portal suspension functionality.
Array of rows, returned by the server, wich all share one row description. Simple queries may include multiple SQL statements, each corresponding to row block. In extended query protocol flow, row block is retuned for each "Execute" message.