Index: firmware/App/Services/DDInterface.c
===================================================================
diff -u -r222b05e39082f727dcfa2d144fa6e040501882f0 -r52787cd53986cfd0abb7e674bf096898e5070f9a
--- firmware/App/Services/DDInterface.c	(.../DDInterface.c)	(revision 222b05e39082f727dcfa2d144fa6e040501882f0)
+++ firmware/App/Services/DDInterface.c	(.../DDInterface.c)	(revision 52787cd53986cfd0abb7e674bf096898e5070f9a)
@@ -7,8 +7,8 @@
 *
 * @file    DDInterface.c
 *
-* @author  (last)      Raghu Kallala
-* @date    (last)      01-May-2026
+* @author  (last)      Varshini Nagabooshanam
+* @date    (last)      15-Jun-2026
 *
 * @author  (original)  Sean Nash
 * @date    (original)  01-Aug-2024
@@ -122,6 +122,7 @@
     dialysateDeliveryCmdSet.bicarbConvFactor  = 0.0F;
     dialysateDeliveryCmdSet.sodium            = 0;
     dialysateDeliveryCmdSet.bicarbonate       = 0;
+    dialysateDeliveryCmdSet.substitutionRate  = 0.0F;
 }
 
 /**********************************************************************//**
@@ -430,6 +431,7 @@
  *************************************************************************/
 void cmdStartGenerateDialysate( F32 qd, F32 quf, F32 dialTemp, U32 genDialysateState, F32 acidConvFactor, F32 bicarbConvFactor, U32 sodium, U32 bicarbonate )
 {
+
     dialysateDeliveryCmdSet.start             = TRUE;
     dialysateDeliveryCmdSet.dialRate          = qd;
     dialysateDeliveryCmdSet.ufRate            = quf;
@@ -439,6 +441,7 @@
     dialysateDeliveryCmdSet.bicarbConvFactor  = bicarbConvFactor;
     dialysateDeliveryCmdSet.sodium            = sodium;
     dialysateDeliveryCmdSet.bicarbonate       = bicarbonate;
+    dialysateDeliveryCmdSet.substitutionRate = 0.0F;
 
 #ifndef TEST_UI_ONLY
     sendMessage( MSG_ID_DD_GEN_DIALYSATE_REQUEST_DATA, COMM_BUFFER_OUT_CAN_TD_2_DD, (U08*)(&dialysateDeliveryCmdSet), sizeof( DIALYSATE_DELIVERY_REQ_PAYLOAD_T ) );
@@ -652,7 +655,33 @@
 //    }
 }
 
+/*********************************************************************//**
+ * @brief
+ * The cmdSubstitutionRate function sends a generate dialysate command to
+ * the DD with a given substitution (D92) pump rate for HDF / HD-online
+ * fluid bolus delivery.
+ * @details \b Message \b Sent: MSG_ID_DD_GEN_DIALYSATE_REQUEST_DATA
+ * @details \b Inputs: none
+ * @details \b Outputs: dialysateDeliveryCmdSet
+ * @param qs Target substitution flow rate (mL/min)
+ * @return none
+ *************************************************************************/
+void cmdSubstitutionRate( F32 qs )
+{
+    if ( TRUE == dialysateDeliveryCmdSet.start )
+    {
+        dialysateDeliveryCmdSet.substitutionRate = qs;
+#ifndef TEST_UI_ONLY
+        sendMessage( MSG_ID_DD_GEN_DIALYSATE_REQUEST_DATA, COMM_BUFFER_OUT_CAN_TD_2_DD, (U08*)(&dialysateDeliveryCmdSet), sizeof( DIALYSATE_DELIVERY_REQ_PAYLOAD_T ) );
+#endif
+    }
+    else
+    {
+        // TODO - s/w fault?
+    }
+}
 
+
 /*************************************************************************
  *                       TEST SUPPORT FUNCTIONS
  *************************************************************************/