diff --git a/doc/rtd/explanation/format/mime.rst b/doc/rtd/explanation/format/mime.rst
index 829367eb964..f5ecfa040bc 100644
--- a/doc/rtd/explanation/format/mime.rst
+++ b/doc/rtd/explanation/format/mime.rst
@@ -77,4 +77,27 @@ Create user-data containing 3 shell scripts:
 
     $ cloud-init devel make-mime -a always.sh:x-shellscript-per-boot -a instance.sh:x-shellscript-per-instance -a once.sh:x-shellscript-per-once
 
+Per-frequency shell scripts
+---------------------------
+
+Cloud-init supports three MIME content types for controlling how often a
+shell script runs, corresponding to the run frequencies described in
+:ref:`module_frequency`:
+
+``text/x-shellscript-per-boot``
+    The script runs on **every boot**. Useful for tasks that must be
+    repeated each time the instance starts, such as mounting ephemeral
+    storage or refreshing dynamic configuration.
+
+``text/x-shellscript-per-instance``
+    The script runs **once per instance-id**. A new instance-id is
+    assigned each time a new instance is provisioned.
+
+``text/x-shellscript-per-once``
+    The script runs **once**, tracked by a semaphore in
+    :file:`/var/lib/cloud/sem`. If cloud-init state is preserved — for
+    example, when an image is created from a running instance — the
+    semaphore persists and the script will not run again on instances
+    booted from that image.
+
 .. _make-mime: https://github.com/canonical/cloud-init/blob/main/cloudinit/cmd/devel/make_mime.py