diff --git a/api.bs b/api.bs
index 8f29ff2..9961184 100644
--- a/api.bs
+++ b/api.bs
@@ -1598,7 +1598,7 @@ after the [=common matching logic=] is applied and privacy budgeting occurs.
 To <dfn>do attribution and fill a histogram</dfn>, given
   [=validated conversion options=] |options|,
   [=site=] |topLevelSite|,
-  [=site=] |intermediarySite|,
+  [=site=] or `undefined` |intermediarySite|,
   and [=moment=] |now|:
 
 1.  Let |matchedImpressions| be an [=set/is empty|empty=] [=set=].
@@ -1688,7 +1688,7 @@ To <dfn>create an all-zero histogram</dfn>, given an integer |size|:
 To perform <dfn>common matching logic</dfn>, given
 [=validated conversion options=] |options|,
 [=site=] |topLevelSite|,
-[=site=] |intermediarySite|,
+[=site=] or `undefined` |intermediarySite|,
 [=epoch index=] |epoch|, and [=moment=] |now|:
 
 1.  Let |matching| be an [=set/is empty|empty=] [=set=].
@@ -1716,11 +1716,11 @@ To perform <dfn>common matching logic</dfn>, given
         and [=set/contains|does not contain=] |topLevelSite|,
         [=iteration/continue=].
 
-    1.  If |intermediarySite| is not `undefined`, let |caller| be |intermediarySite|.
-        Otherwise, let |caller| be |topLevelSite|.
+    1.  Let |conversionCaller| be |intermediarySite| if it is not `undefined`,
+        |topLevelSite| otherwise.
 
     1.  If |impression|'s [=impression/conversion callers=] [=set/is empty|is not empty=]
-        and [=set/contains|does not contain=] |caller|,
+        and [=set/contains|does not contain=] |conversionCaller|,
         [=iteration/continue=].
 
     1.  If |options|' [=validated conversion options/match values=] [=set/is empty|is not empty=]
@@ -1732,9 +1732,11 @@ To perform <dfn>common matching logic</dfn>, given
         [=list/contains|does not contain=] |impression|'s [=impression/impression site=],
         [=iteration/continue=].
 
+    1.  Let |impressionCaller| be |impression|'s [=impression/intermediary site=]
+        if it is not `undefined`, |impression|'s [=impression/impression site=] otherwise.
+
     1.  If |options|' [=validated conversion options/impression callers=] [=set/is empty|is not empty=]
-        and [=set/contains|does not contain=] |impression|'s [=impression/intermediary site=]
-        or |impression|'s [=impression/impression site=],
+        and [=set/contains|does not contain=] |impressionCaller|,
         [=iteration/continue=].
 
     1.  [=set/Append=] |impression| to |matching|.
diff --git a/impl/src/backend.ts b/impl/src/backend.ts
index 1b33ae4..5416308 100644
--- a/impl/src/backend.ts
+++ b/impl/src/backend.ts
@@ -392,10 +392,10 @@ export class Backend {
       ) {
         continue;
       }
-      let caller = intermediarySite ?? topLevelSite;
+      const conversionCaller = intermediarySite ?? topLevelSite;
       if (
         impression.conversionCallers.size > 0 &&
-        !impression.conversionCallers.has(caller)
+        !impression.conversionCallers.has(conversionCaller)
       ) {
         continue;
       }
@@ -408,10 +408,12 @@ export class Backend {
       ) {
         continue;
       }
-      // TODO: The wording from Step 4.10 of
-      // https://w3c.github.io/ppa/#common-matching-logic is a bit ambiguous.
-      caller = impression.intermediarySite ?? impression.impressionSite;
-      if (impressionCallers.size > 0 && !impressionCallers.has(caller)) {
+      const impressionCaller =
+        impression.intermediarySite ?? impression.impressionSite;
+      if (
+        impressionCallers.size > 0 &&
+        !impressionCallers.has(impressionCaller)
+      ) {
         continue;
       }
       matching.add(impression);