diff --git a/core/src/main/java/org/lflang/generator/ReactionInstanceGraph.java b/core/src/main/java/org/lflang/generator/ReactionInstanceGraph.java
index 5ce3ff38fb..ceb2404538 100644
--- a/core/src/main/java/org/lflang/generator/ReactionInstanceGraph.java
+++ b/core/src/main/java/org/lflang/generator/ReactionInstanceGraph.java
@@ -223,7 +223,11 @@ protected void addNodesAndEdges(ReactorInstance reactor) {
     registerPortInstances(reactor);
   }
 
-  /** Add edges that encode the precedence relations induced by the TPO levels. */
+  /**
+   * Add edges that encode the precedence relations induced by the TPO levels. TPO is total port
+   * order. See
+   * https://github.com/icyphy/lf-pubs/blob/54af48a97cc95058dbfb3333b427efb70294f66c/federated/TOMACS/paper.tex#L1353
+   */
   private void addEdgesForTpoLevels(ReactorInstance main) {
     var constrainedReactions = getConstrainedReactions(main);
     for (var i : constrainedReactions.keySet()) {
diff --git a/core/src/main/java/org/lflang/generator/ReactorInstance.java b/core/src/main/java/org/lflang/generator/ReactorInstance.java
index 39f7cd03da..db4b301d2d 100644
--- a/core/src/main/java/org/lflang/generator/ReactorInstance.java
+++ b/core/src/main/java/org/lflang/generator/ReactorInstance.java
@@ -181,7 +181,8 @@ public ReactorInstance(Reactor reactor, ReactorInstance parent, MessageReporter
 
   /**
    * The TPO level with which {@code this} was annotated, or {@code null} if there is no TPO
-   * annotation.
+   * annotation. TPO is total port order. See
+   * https://github.com/icyphy/lf-pubs/blob/54af48a97cc95058dbfb3333b427efb70294f66c/federated/TOMACS/paper.tex#L1353
    */
   public final Integer tpoLevel;