+A hash reference of additional string substitutions
+
+=item sub_param
+
+A hash reference, keys are the names of existing substitutions,
+values are an addition parameter object to pass to the subroutine
+for that substitution, e.g.
+
+ 'sub_param' => {
+ 'payment_history' => {
+ 'start_date' => 1434764295,
+ },
+ },