Lets suppose 2 documents are added to the _replicator database in the following order:

{
    "_id": "doc_A",
    "source":  "http://myserver.com:5984/foo",
    "target":  "bar"
}

and

{
    "_id": "doc_B",
    "source":  "http://myserver.com:5984/foo",
    "target":  "bar"
}

Both describe exactly the same replication (only their _ids differ). In this case document doc_A triggers the replication, getting updated by CouchDB with the fields _replication_state, _replication_state_time and _replication_id, just like it was described before. Document doc_B however, is only updated with one field, the _replication_id so it will look like this:

{
    "_id": "doc_B",
    "source":  "http://myserver.com:5984/foo",
    "target":  "bar",
    "_replication_id":  "c0ebe9256695ff083347cbf95f93e280"
}

While document doc_A will look like this:

{
    "_id": "doc_A",
    "source":  "http://myserver.com:5984/foo",
    "target":  "bar",
    "_replication_id":  "c0ebe9256695ff083347cbf95f93e280",
    "_replication_state":  "triggered",
    "_replication_state_time":  1297974122
}

Note that both document get exactly the same value for the _replication_id field. This way you can identify which documents refer to the same replication - you can for example define a view which maps replication IDs to document IDs.