diff --git a/synapse/storage/schema/delta/55/add_spans_to_device_lists.sql b/synapse/storage/schema/delta/55/add_spans_to_device_lists.sql
index d4c09005ab..bbd5d58486 100644
--- a/synapse/storage/schema/delta/55/add_spans_to_device_lists.sql
+++ b/synapse/storage/schema/delta/55/add_spans_to_device_lists.sql
@@ -13,4 +13,11 @@
  * limitations under the License.
  */
 
+/*
+ * Opentracing needs to inject a span_context into data item which leaves the
+ * current execution context. Since device list updates are dumped in here
+ * and then processed later they need to include the span context for opentraing.
+ * Since we may also decide later to include other tracking information the column
+ * has just been called "context", the structure of the data within it may change.
+ */
 ALTER TABLE device_lists_outbound_pokes ADD context TEXT;
\ No newline at end of file