diff --git a/doc/examples/kea6/advanced.json b/doc/examples/kea6/advanced.json
index 6730c9e9b8..b82d6016eb 100644
--- a/doc/examples/kea6/advanced.json
+++ b/doc/examples/kea6/advanced.json
@@ -67,6 +67,10 @@
     {
       "pools": [ { "pool": "2001:db8:1::/80" } ],
 
+# This defines PD (prefix delegation) pools. In this case
+# we have only one pool. That consists of /64 prefixes
+# being delegated out of large /48 pool. Each delegated
+# prefix will contain an excluded-prefix option.
       "pd-pools": [
       {
           "prefix": "2001:db8:abcd::",
diff --git a/src/bin/dhcp6/dhcp6_parser.yy b/src/bin/dhcp6/dhcp6_parser.yy
index 72b03d9edc..2bf1cc7cd7 100644
--- a/src/bin/dhcp6/dhcp6_parser.yy
+++ b/src/bin/dhcp6/dhcp6_parser.yy
@@ -933,15 +933,20 @@ sub_option_data: LCURLY_BRACKET {
 };
 
 // This defines parameters specified inside the map that itself
-// is an entry in option-data list.
+// is an entry in option-data list. It can either be empty
+// or have a non-empty list of parameters.
 option_data_params: %empty
                   | not_empty_option_data_params
                   ;
 
+// Those parameters can either be a single parameter or
+// a list of parameters separated by comma.
 not_empty_option_data_params: option_data_param
     | not_empty_option_data_params COMMA option_data_param
     ;
 
+// Each single option-data parameter can be one of the following
+// expressions.
 option_data_param: option_data_name
                  | option_data_data
                  | option_data_code