diff --git a/docs/en/serverless/alerting/create-custom-threshold-alert-rule.asciidoc b/docs/en/serverless/alerting/create-custom-threshold-alert-rule.asciidoc
index 536a2d3b9a..078f57d94b 100644
--- a/docs/en/serverless/alerting/create-custom-threshold-alert-rule.asciidoc
+++ b/docs/en/serverless/alerting/create-custom-threshold-alert-rule.asciidoc
@@ -99,6 +99,8 @@ Set an equation and threshold to trigger an alert when the number of stateful po
 The preview chart provides a visualization of how many entries match your configuration.
 The shaded area shows the threshold you've set.
 
+If you use the **Group alerts by** option, the maximum bar size will be 3. It will only show the top 3 fields.
+
 [discrete]
 [[custom-threshold-group-by]]
 == Group alerts by (optional)