A generic Replication Pipeline is illustrated below.
Outbound Processing Service
Outbound Processing Service is responsible for capturing and recording Replication Events that occur in the Source Repository. Outbound Processing Service is controlled by Replication Maps which determine what Events need to be captured, packaged, and transferred to the Target Repository. Groups of Replication Events are packaged into two types of messages or objects: Queued Items and Replication Packages.
A Queued Item is a unit of work to be transferred to a Target Repository for remote execution. The Replication Web Service on a Target Repository is called to push a Queued Item from the Source Repository to a Target Repository.
A Replication Package is a collection of one or more Replication Events plus data about the changed information that is packaged in a format specific to the Replication Transport being used. When an Event is being processed, Outbound Processing Service calls the Source repository object model to extract the changed information from the Source Repository.
Package Transfer Service
The Package Transfer Service activity is responsible for the transfer and receipt of Queued Items and the downloading of Replication Packages (Packages) from the Source Repository to the Target Repository. Package Transfer is the process that sits between Outbound Processing Service (on the Source Repository) and Inbound Processing Service (on the Target Repository).
Inbound Processing Service
Inbound Processing Service is responsible for processing the Queued Items and Packages received and accepted by the Target Repository and applying them to the repository. The Queued Items and Packages are applied to the Target Repository content base by calling the Target Repository object model.