diff --git a/test/units/testsuite-15.sh b/test/units/testsuite-15.sh
index 0266f6266fb..204fa992199 100755
--- a/test/units/testsuite-15.sh
+++ b/test/units/testsuite-15.sh
@@ -8,6 +8,7 @@ set -o pipefail
 # shellcheck source=test/units/util.sh
 . "$(dirname "$0")"/util.sh
 
+# Needed to write units to /usr/lib/systemd/system to test /etc and /run overrides.
 maybe_mount_usr_overlay
 trap 'maybe_umount_usr_overlay' EXIT
 
diff --git a/test/units/testsuite-26.sh b/test/units/testsuite-26.sh
index 2dd62a4f674..6734aee654f 100755
--- a/test/units/testsuite-26.sh
+++ b/test/units/testsuite-26.sh
@@ -18,6 +18,7 @@ at_exit() {
     return 0
 }
 
+# Needed for /usr/lib/systemd/system/$UNIT_NAME to test overrides in /etc and /run
 maybe_mount_usr_overlay
 trap at_exit EXIT
 
diff --git a/test/units/testsuite-60.sh b/test/units/testsuite-60.sh
index 3d0723e7dcc..c777de85263 100755
--- a/test/units/testsuite-60.sh
+++ b/test/units/testsuite-60.sh
@@ -6,6 +6,7 @@ set -o pipefail
 # shellcheck source=test/units/util.sh
 . "$(dirname "$0")"/util.sh
 
+# Needed to create mount.mytmpfs helper
 maybe_mount_usr_overlay
 trap 'maybe_umount_usr_overlay' EXIT
 
diff --git a/test/units/testsuite-73.sh b/test/units/testsuite-73.sh
index 3e5b78879ad..59f84f20abc 100755
--- a/test/units/testsuite-73.sh
+++ b/test/units/testsuite-73.sh
@@ -9,6 +9,7 @@ set -o pipefail
 # shellcheck source=test/units/util.sh
 . "$(dirname "$0")"/util.sh
 
+# Needed to generate test locales in /usr/lib
 maybe_mount_usr_overlay
 trap 'maybe_umount_usr_overlay' EXIT