diff --git a/docs/features/software-catalog/descriptor-format.md b/docs/features/software-catalog/descriptor-format.md
index 1c95cfeb2a..f14f211ca9 100644
--- a/docs/features/software-catalog/descriptor-format.md
+++ b/docs/features/software-catalog/descriptor-format.md
@@ -1269,6 +1269,15 @@ but there will always be one ultimate owner.
 | ------------------------------------------------------ | ------------------------------------------ | ------------------------------------------------------------------------------- |
 | [`Group`](#kind-group) (default), [`User`](#kind-user) | Same as this entity, typically `default`   | [`ownerOf`, and reverse `ownedBy`](well-known-relations.md#ownedby-and-ownerof) |
 
+### `spec.subdomainOf` [optional]
+
+An [entity reference](references.md#string-references) to another domain of
+which the domain is a part, e.g. `audio`. This field is optional.
+
+| [`kind`](#apiversion-and-kind-required) | Default [`namespace`](#namespace-optional) | Generated [relation](well-known-relations.md) type                            |
+| --------------------------------------- | ------------------------------------------ | ----------------------------------------------------------------------------- |
+| [`Domain`](#kind-domain)                | Same as this entity, typically `default`   | [`partOf`, and reverse `hasPart`](well-known-relations.md#partof-and-haspart) |
+
 ## Kind: Location
 
 Describes the following entity kind: