It is explicitly mentionned in the configuration manual that the parent of a
SPOE stream is the filtered stream. It means variables of the filtered
stream are usable from the SPOE stream.
SPOE communicates with external components using an in-house binary protocol,
the Stream Processing Offload Protocol (SPOP).
+When the SPOE is used on a stream, a dedicated stream is spawned to handle the
+communcation with the external component. The main stream is the parent stream
+of this "SPOE" stream. It means it is possible to retrieve variables of the
+main stream from the "SPOE" stream. See section 2.8 about variables for
+details.
+
For all information about the SPOE configuration and the SPOP specification, see
"doc/SPOE.txt".