diff --git a/schema/nmdc.yaml b/schema/nmdc.yaml
index 1e4bfea..132a542 100644
--- a/schema/nmdc.yaml
+++ b/schema/nmdc.yaml
@@ -91,6 +91,7 @@ classes:
       - data object type
       - compression type
       - was generated by
+      - url
     slot_usage:
       was generated by:
         description: >-