diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index 3de11f0..570eb2d 100644
--- a/src/include/nodes/primnodes.h
+++ b/src/include/nodes/primnodes.h
@@ -266,7 +266,20 @@ typedef struct Param
  * replaced with a single argument representing the partial-aggregate
  * transition values.
  *
- * XXX need more documentation about partial aggregation here
+ * 'aggcombine' and 'aggpartial' define what the expected input values and
+ * what the expected output value for the aggregate will be. By default both
+ * of these are False, and are only ever True when aggregation must take place
+ * in multiple stages, such as parallel aggregation. In this case the
+ * aggregate value will be calculated in two stages. The first stage will take
+ * normal input values and aggregate them, but not call the final function on
+ * any of these values. Instead the transition state is returned to the
+ * caller. Such a setup will require that the 2nd executor node will perform
+ * the finalization of the aggregate, and such a node would be set up to have
+ * 'aggpartial' set to False, and would have 'aggcombine' set to True (since
+ * the expected input values are expected to be transition states, rather than
+ * normal input values). It is also possible for aggregation to take place in
+ * more than two stages, where an intermediate combine may leave values to be
+ * finalized by another executor node.
  *
  * 'aggtype' and 'aggoutputtype' are the same except when we're performing
  * partal aggregation; in that case, we output transition states.  Nothing