diff --git a/contributed_definitions/NXxpcs.nxdl.xml b/contributed_definitions/NXxpcs.nxdl.xml
index 3024dd7d1..4693b7485 100644
--- a/contributed_definitions/NXxpcs.nxdl.xml
+++ b/contributed_definitions/NXxpcs.nxdl.xml
@@ -57,15 +57,32 @@
- Unique identifier for the experiment.
+ **Locally** unique identifier for the experiment (a.k.a. run or scan).
+
+ * For bluesky users, this is the run's `"scan_id"`.
+ * For SPEC users, this is the scan number (``SCAN_N``).
(optional) UUID identifier for this entry.
+
+ See the `UUID standard <https://www.rfc-editor.org/rfc/rfc4122.html>`__ (or
+ `wikipedia <https://en.wikipedia.org/wiki/Universally_unique_identifier>`__)
+ for more information.
+
+ * For `bluesky <https://blueskyproject.io/>`__ users, this is the
+ run's `"uid"` and is expected for that application.
+ * Typically, `SPEC <https://certif.com/content/spec/>`__ users will
+ not use this field without further engineering.
-
+
+
Scan number (must be an integer).