diff --git a/capabilities/README.md b/capabilities/README.md
index 73588de..ed3c718 100644
--- a/capabilities/README.md
+++ b/capabilities/README.md
@@ -251,6 +251,19 @@ It MAY be used in place of the file with the same name in the AMWA BCP-004-01 sp
   - **Target:** (a) Sender `st2110_21_sender_type` (defined in the [Sender Attributes](../sender-attributes/#st-2110-21-sender-type) register), (b) SDP attribute `a=fmtp:` format-specific parameter `TP`
 - **Applicability:** AMWA IS-04
 
+## Substream Constraints
+
+Mux Senders create IS-04 Flow objects for each subflow included in the mux Flow.
+The mux Flow links to each subflow object using the IS-04 [`parents`](https://specs.amwa.tv/is-04/latest/docs/Data_Model_-_Identifier_Mapping.html#source-and-flow-representation) attribute.
+
+### Substreams
+- **Name:** `urn:x-nmos:substreams`
+- **Description:** Identifies any per substream constraints.
+- **Specification:** per AMWA BCP-004-01
+  - **Type:** array of objects (as per [AMWA BCP-004-01](https://specs.amwa.tv/bcp-004-01/))
+  - **Target:** mux Sender subflows and their associated attributes targeted by individual Substream Parameter Constraints
+- **Applicability:** AMWA IS-04
+
 [RFC-4566]: https://tools.ietf.org/html/rfc4566 "SDP: Session Description Protocol"
 [RFC-9134]: https://tools.ietf.org/html/rfc9134 "RTP Payload Format for ISO/IEC 21122 (JPEG XS)"
 [color-sampling]: https://www.iana.org/assignments/media-type-sub-parameters/media-type-sub-parameters.xhtml#media-type-sub-parameters-15 "Media Type Sub-Parameter Registry for video/raw: Color (sub-)sampling"
diff --git a/capabilities/constraint_set.json b/capabilities/constraint_set.json
index c662d51..553739d 100644
--- a/capabilities/constraint_set.json
+++ b/capabilities/constraint_set.json
@@ -18,6 +18,13 @@
       "type": "boolean",
       "default": true
     },
+    "urn:x-nmos:substreams": {
+      "description": "This exposes any substream constraints.",
+      "type": "array",
+      "items": {
+        "$ref": "substream_constraints.json"
+      }
+    },
     "urn:x-nmos:cap:format:media_type": {
       "$ref": "param_constraint_string.json"
     },